Here are the business and investment highlights of the Economic Recovery Tax Act of 1981, which President Reagan will sign into law this week. The information was gathered by Washington Post staff writers Thomas B. Edsall, Caroline Atkinson and Jerry Knight from the congressional conference report on the bill and the summary of the law prepared by the congressional Joint Committee on Taxation. For changes in the real estate tax procisions, see Saturday's Real Estate section . ACCELERATED COST-RECOVERY SYSTEM

Accelerated and simplified depreciation rules would put almost all investments made after Dec. 31 into one of four main categories for writing off in 15, 10, 5 or 3 years. This would replace the old "useful life" concept that aimed to allocate depreciation deductions over the period in which the asset was used.

The categories are as follows, with optional longer depreciation periods for all items in each category in parentheses.

Three years -- autos, light-duty trucks, research and development equipment, racehorses over 2 years old, other horses over 12 years old. (5, 12 years)

Five years -- most other equipment except long-lived public utility property. (12, 25 years)

Ten years -- public utility property with previously estimated depreciation life of between 19 and 25 years (25, 35 years)

Fifteen years -- public utility property with previous depreciation life of more than 25 years, real property, (35, 45 years)

Taxpayers may use straight-line depreciation or accelerated depreciation on the following schedule:

For 1981-84, the 150-percent-declining-balance method, rising to 175 percent for 1985, and 200 percent, or double-declining balance, thereafter.

Taxpayers electing to write off over longer periods must use straight-line depreciation. Low-income housing may be written off using the 200 percent declining-balance method, changing to straight-line. Other real property may use 175 percent, changing to straight-line to maximize depreciation.

Investment tax credits would be 10 percent for all eligible investments in the 5-year, 10-year and 15-year write-off categories, and 6 percent for investments in the 3-year category. Eligivle properties are tangible personal property, and other tangible property used in connection with manufacturing, production and some other activities, but not distribution. It now includes facilities for storing petroleum.

Credits would be applicable to used property of up to $125,000 for 1981-84, and $150,000 thereafter.

If the asset is sold, a 2 percent credit is allowed for each full year the property is held and any excess is recaptured by the IRS.

Unused credits may be carried forward and backward for 15 years compared with 3 years forward and 7 back under the old law. But this will not be retroactive for investment tax credits unused before 1973.

A new rule will limit eligibility for investment tax credits to those "at risk." Investments in business activities that are financed by nonrecourse loans generally won't quality for tax credits. This is already the case for depreciation allowances.

A major change will let many more transactions be characterized as leasing if the lessor and lessee elect to treat the lessor as the property owner. This will allow less-profitable companies, which otherwise could not use their full depreciation allowances and tax credits, to lease equipment from companies which can use the deductions and credits. The tax benefits can be shared, between lessor and lessee, through varying the leasing charges.

This will be allowed even if the lessor only makes a profit because of the tax benefits; the lessee retains title and is for practical purposes like the owner of the equipment; the lessee provides financing for the investment, other than a minimum 10 percent stake by the lessor; the lessee may not use the property after the end of the lease; the property may be purchased after the end of the lease at more or less than a fair market value; and an obligatioon of any person in the arrangement is subject to a contingency or offset agreement. RESEARCH DEVELOPMENT AND EXPERIMENTATION

Corporations will be given a 25 percent tax credit for research and experimentation costs. The credit would be limited, however, to the extent that the expenditures exceed previous investments in the field. The base period for calculating this amount would be the average over the three previous years. In addition, corporations giving new scientific equipment to colleges and universities can claim a charitable deduction equal to the taxpayer's basis plus 50 percent of the appreciation. In a tax break benefiting primarily drug and electronics firms, corporate taxpayers will be allowed for two years to allocate research and experimentation costs conducted in the United Stats entirely to U.S.-source income. SMALL BUSINESS

In general, small business did not do as well under the legislation passed as it would have under the Democratic alternative bill rejected by the House. Under the enacted legislation, the corporate income tax in the two lowest brackets will be reduced by two percentage points. The rate on income below $25,000 will drop from 17 percent to 16 percent in 1982 and to 15 percent in 1983. For income between $25,000 and $50,000, the rate will drop from 20 percent to 19 percent in 1982 and to 18 percent in 1983.

In addition to the changed depreciation schedules under the sections known as "10-5-3," business would be allowed to write off in one year the cost of up to $5,000 in new or used property in 1982 and 1983, $7,500 in 1984 and $10,000 from 1985 onwards.

Among the other minor small-business sections are an increase in the accumulated earnings credit -- the amount corporations can retain to cover "reasonable" needs before distributing dividends -- from $150,000 to $250,000, and an increase in the number of shareholders permitted in Subchapter S corporations from 15 to 25. WINDFALL OIL TAX

The tax bill provides a total of $11.8 billion in tax breaks to various segments of the oil industry over the next five years. Oil produced from stripper well -- those that produce 10 or fewer barrels a day -- owned by independent producers will be exempt from the tax starting in 1983. Royalty owners will have the current $1,000 tax credit raised to $2,500 this year; from 1982 through 1984, 2 barrels of oil a day will be exempt from the tax (the equivalent of a $8,600 credit), and in 1985 this will grow to a 3-barrel exemption, or the equivalent of a $12,900 credit. The 30 percent rate on newly discovered oil will drop by 2 1/2 percentage point steps each year until it reaches 15 percent in 1986. SAVINGS AND INVESTMENT INCENTIVES

In one of the more controversial sections of the bill, lending institutions will be permitted to issue special certificates from this Oct. 1 through Dec. 31, 1982. Purchasers of the certificates would be allowed to receive tax-free income of $1,000 for a single return or $2,000 on a joint return. The interest rate would be set at 70 percent of the yield on 52-week Treasury bills. The certificates are advantageous only to persons in upper-income tax brackets.

The lending institution would be requird to use 75 percent of the proceeds of the certificates for residential financing, a move designed to boost home building and to help the beleaguered savings and loan industry.

The current $200 exclusion for single returns and $400 for couples for interest and divident income is repealed. Taxpayers will be allowed to exclude only $100 of divident income for a single return and $200 for a joint return.

In an effort to prevent Congress from continuing the savers' certificate program, the Treasury persuaded Congress to adopt a new interest exclusioin starting in 1985: 15 percent of net interest income can be excluded, up to a ceiling of $450 for single returns and $900 on joint returns.

The new system will be far more advantageous to persons with very large amounts of savings and provide little tax relief to those with relatively small accounts. Under current law, a person with $2,000 earning 10 percent interest could exclude the entire $200 in interest. After 1984, a person would have to have $13,333 in the bank at 10 percent rates in order to get the same $200 exclusion.

The amount of money a taxpayer can put into an individual retirement account (IRA) and postpone tax liability on will grow from $1,500 to $2,000, and all persons, including those now covered by company pension plans, will be able to participate in IRAs. The maximum annual deduction for contributions to Keogh self-employed retirement plans will increase from $7,500 to $15,000.

The investment tax credit for contributions to employe stock ownership plans (ESOPs) will be terminated in 1982 and replaced with a payroll credit. For 1983 and 1984, the credit will be one-half percent of compensation for employes, growing to three-quarters percent in 1985 and expiring in 1987. The purpose of the shift to payroll-based credits is to encourage ESOPs in labor-intensive businesses.

Domestic public utilities will be permitted to issue stock as dividends (up to an annual ceiling of $750 for the recipient filing a single return, $1,500 on a joint return) so that if the stock is held for over a year, the recipient can be taxed at more favorable capital gains rates instead of ordinary income rates applied to cash dividends.

In a significant alteration of law governing stock incentive programs, employes exercising stock options no longer will be subject to income tax before the stock is sold. Instead, the option, which must be granted at 100 percent of market price, can be exercised without tax liability until it is sold, when it will be subject to taxation at capital gains rates. CAPITAL GAINS

The maximum tax on long-term capital gains is reduced retroactively from 28 to 20 percent for sales and exchanges after June 9, 1981. The maximum tax on unearned or property income, including interest and divident income, will drop from 70 to 50 percent effective Jan. 1, 1982. ESTATE AND GIFT TAXES

The exemption on estates will grow from the current $175,625 to $225,000 in 1982, to $275,000 in 1983, to $325,000 in 1984, to $400,000 in 1985, to $500,000 in 1986 and to $600,000 in 1987. The maximum rate on the value of estates in excess of $2 million will drop from 70 percent to 65 percent in 1982, to 60 percent in 1983, to 55 percent in 1984 and to 50 percent in 1985. There will be no tax whatsoever on estates going to spouses. The current use valuation will be increased from $500,000 to $600,000 this year, to $700,000 in 1982 and to $750,000 in 1983 and thereafter, in a change advantageous to farmers. The gift tax exclusion will be raised from $3,000 to $10,000 starting in 1982. FOREIGN EARNED INCOME

The current complex system of deductions will be changed to an exclusion with a maximum of $75,000 starting in 1982 and growing in $5,000 steps to $95,000 in 1986. In addition, housing costs in excess of about $6,100 can ber excluded starting in 1982. Jobs credit

The targeted jobs credit program is extended for one year to eligible employes starting work before 1983. For Vietnam War veterans, the age limit of 35 is lifted, and persons laid off from the public service CETA job program are eligible to participate. COMMODITY STRADDLES

Commodity futures traders no longer can use a technique known as a commodity straddle to defer income from one year to the next or to convert ordinary income into capital gains taxed at a lower rate.

Effective June 23, it is no longer necessary to hold commodity investments for six months to quality for long-term capital gains treatment. Instead, all commodity trading profits will be taxed at an effective rate of 32 percent.

The rules for writing off losses on Treasury bills are changed. Interest and carrying charges on commodity investments no longer can be deducted directly but must be added to the cost of the investment. MISCELLANEOUS

Trucking companies that suffered losses in operating rights as a result of deregulation legislation will be permitted to amortize the basis of those rights over a 60-month period. Corporations whose income exceeds $1 million in any of three preceding tax years will have to increase estimated tax payments from 60 percent to 65 percent in 1982, to 75 percent in 1983 and to 80 percent thereafter. The prohibition against issuing regulations governing taxation of fring benefits will be extended through 1983.