The $18.7 billion tax bill Congres passed as its final act yesterday afternoon marks a major departure from other tax cuts by Democratic congresses in the past 15 years. Most of the benefits would go to taxpayers in the upper-income tax brackets.

Although the lawmakers did in the end provide additional relief for the lower-income brackets, the 50 percent of the taxpayers who get $15,000 a year or less still receive only 21 percent of the bill's total tax cuts for individuals. The other half making $15,000 and up will get 79 percent.

In all, the measure contains $13.1 billion in reductions for individuals, providing an average 7.2 percent tax cut - most of it concentrated in the $15,000 to $50,000 range.A family of four making $15,000 a year would receive a tax cut of $167.

For the broad "middle class" in the $20,000 to $30,000 brackets, the reductions will be just enough to offset next January's big Social Security tax increases and the impact of a year's inflation on the income tax brackets. But for most lower and higher brackets, it won't be sufficient.

The massive legislation, sharply trimmed back from the $29.1 billion version approved by the Senate earlier, was formally approved and sent to President Carter after a conference committee struggled in closed-door caucuses late Saturday and early yesterday to work out a compromise.

The Senate passed the bill yesterday morning by a vote of 72 to 3, after defeating a last-ditch GOP motion to revive the Democratic version of the Roth-Kemp tax-cut plan sponsored by Sen. Sam Nunn (D-Ga.). The House approved the measure 337 to 38.

Although the White House declined to say yesterday whether the president would sign the bill. Stuart E. Eizenstat, Carter's chief domestic adviser, noted that the conferees had stripped the bill of most of the administrative's objections - making a veto far less likely.

Not only was the final version well within bounds of the president's $21 billion budgetary target, but the conferees also included extra tax cuts for lower-income families, reduced the size of the cuts in capital gains taxes and patched up a proposal to dilute the "minimum tax."

The bill contains these major elements:

$13.1 billion in tax cuts for individuals, including an increase in the $750 personal exemption to $1,000 for each taxpayer and dependent: a boost in the standard deduction or "zero bracket amount," and across-the-board reductions in tax rates.

The lawmakers also increased the "earned income credit" for the working poor to a maximum of $500 from $400 now and made partial credits available to families earning up to $10,000. The ceiling is $8,000 now. Those too poor to benefit from a tax credit would get cash payments through the withholding system.

Repeal of the federal income tax deduction for state and local gasoline taxes. The measure was proposed by Carter as a way to help simplify the income tax return. It also would raise an estimated $2 billion for the treasury.

A provision allowing homeowners 55 and older a one-time opportunity to take the first $100,000 in profits from the sale of their house tax free - a measure designed to aid retirees who want to sell their homes and move to apartments or smaller houses.

The lawmakers also killed a controversial college tuition tax credit propostal that Carter had warned he would veto, even if he had to reject the entire tax bill. And they watered down the Nunn proposal to a simple statement of goals.

A $2.1 billion cut in capital gains taxes, more than three-quarters of which would benefit taxpayers making more than $50,000, by exempting 60 percent of the profits from the sale of stocks or property from regular income taxes rather than the 50 percent excluded now.

The lawmakers also voted to dilute the "minimum tax" enacted in 1969 to prevent high-income investors from escaping payment of taxes entirely. But they rejected a formula opposed by the administration that would have enabled tax-shelter investors to avoid taxes.

$3.6 billion in business tax cuts, stemming from lowering the corporate tax rate to 46 percent, from the present 48 percent and liberalizing the 10 percent investment tax credit to enable companies to use it to offset 90 percent of their other taxes, rather than 50 percent.

A streamlining of the present jobs tax credit, intended to encourage employers to hire the disadvantaged. The provision would allow employers to reduce their taxes by up to $3,000 for each worker the first year and $1,500 the second.

Virtually all the tax cut provisions would take effect Jan. 1, with two major exceptions: the reduction in capital gains taxes would begin Nov. 1 of this year and the homeowners' tax break would apply to any sale made since July 26.

The measure also contains a handful of token "tax reforms" the president had recommended, including repeal of business deduction for the cost of maintaining yachts and hungting lodges and a provision requiring those who make $20,000 or more and receive unemployment benefits to pay income tax on them.

However, the lawmakers rejected the bulk of Carter's "tax reform" package, including his proposal to repeal the big foreign tax breaks granted multinational corporations and his plan to end deduction for the "three-martini" lunch.

And the bill contains a series of narrow-interest amendments that would benefit specific companies or industries, including a provision granting a tax break to the heirs of the Gallo wine estate and others providing benefits for sod farmers and pig-pen operators.

The scaling-back of the tax bill from the one approved earlier by the Senate also brought down the cost of the measure in coming years, which was a key concern to Carter. The president had expressed apprehension that the Senate bill would be a budget-buster.

According to preliminary figures, the bill the lawmakers sent Carter yesterday would drain $19.3 billion from treasury revenues in the current fiscal 1983, compared to $79 billion for the version passed by the Senate.

By far the knottiest problem for the conferees yesterday morning was how to deal with the Nunn amendment. The provision, which would have cut taxes $142 billion between 1980 and 1983 if the government met stringent targets for holding down spending, was difficult for many legislators to oppose.

Conferees finally agreed to a compromise early yesterday that established as a "national policy" that if tht provision's spending targets were met, it was "the intention" of Congress to enact the tax cuts the Nunn bill specified. The conferees finished at about 4 p.m.

The concentration of the tax reductions in the bill toward middle and upper-income taxpayers marks a visiable departure from previous Democratic tax cut efforts. Since the early 1960s, the bulk of the reductions have been given to those at the lower end of the income sale.

However, while the lawmakers made some effort to enlarge the tax cuts for individuals in the low and lower-middle-income levels, the distribution is skewed toward the middle and upper brackets by the sizable cuts in capital gains taxes and the tax break for homesellers.

In effect, the distribution in yesterday's bill conforms to the pattern in which taxes are paid. Recent figures showed the upper half of the nation's taxpayers pay 94 percent of the federal income taxes collected. Studies have shown these groups have been slighted in previous tax cut bills.