Computer makers are always developing faster and more powerful machines, yet their brawn isn't much good without the programs that make computers useful and versatile -- the software.

The hardware is like the human body while the software, containing hundreds of thousands of detailed instructions, is the brain.

With such a critical role in computing, microcomputer software generates huge retail sales -- about $4.6 billion in North America in 1990, the Software Publishers Association says.

Over the past decade, the biggest change in software is greater ease of use, which means it can be used effectively in a short period of time, said Ken Wasch, executive director of the software association.

And just as computer chips have become more powerful, software programs have increasingly needed that muscle to handle newer, more data-intensive programs such as desktop publishing.

Not only have computer programs given users many applications at home, but communications programs and modems allow them to link up with the outside world to tap extensive databases and services.

For example, Prodigy allows subscribers to book flights and rent cars, scan stock quotes and transfer funds, send messages across the country, play games or access news, sports and weather information.

Although a wide variety of software is available, the biggest sellers tend to have a practical business orientation.

The following is a list of the most popular software, ranked by order of sales in 1990, provided by the Software Publishers Association.

Productivity ($1.05 billion). General business programs, scientific and engineering, home productivity, accounting and electronic mail.

Word Processors ($917 million). One of the mainstay uses of personal computers. Programs are continually upgraded and often include add-ons, such as spelling checkers.

Spreadsheets ($701.3 million). They can perform rapid calculations and "what-if" scenarios easily. Newer spreadsheets can also produce graphics from the numbers used. The versatility of early spreadsheets such as Visicalc gave personal computers credibility as a business tool.

Graphics ($583.7 million). From simple pie or bar charts, graphics programs have become more sophisticated and written to take advantage of color monitors. The desire to produce more visually attractive documents at a lower price and less time spurs demand for graphics programs.

Recreation ($355.5 million). More sophisticated color graphics, sounds and higher-resolution monitors make computer games a popular use of personal computers. Games with battle or sports themes are widely available. Recreation software also gives users a breather from more "serious" computer uses.

Databases ($345 million). Database programs derive their appeal from their ability to quickly access, sort, classify and print massive amounts of data. Applications can range from generating a simple mailing list to storing historical claims experience so that insurance companies can establish rates.

Education ($171.9 million). The computer industry has tried to promote computers as important to the development of children and students. Programs such as Math Blaster, Reader Rabbit, PC Globe and Spanish Assistant are examples. Some programs teach users how to learn more complicated programs, such as databases.

Desktop Publishing ($167.5 million). The ability to merge text and graphics in an attractive package has driven demand for desktop publishing software. However, the cost of high-quality laser printers, the software itself and computer hardware that can handle large data loads means desktop publishing is usually restricted to serious users. One of the better software packages on the market, Ventura Publisher, costs about $549.

Integrated ($158.3 million). These "all-in-one" or "package deal" software programs bundle various functions, such as word-processing, database, spreadsheet, graphics and communications capabilities. Examples are Microsoft Works and Ashton-Tate Framework. Other integrated software may be less elaborate, providing business organization tools such as note pads, calendars and calculators.

Languages-Tools ($112 million). The relative lack of popularity of these programs reflects the fact that they aren't for the average computer user. They are intended for programming, debugging, customizing and other such activities in which computer nerds engage.