The most basic definition of open source software is :

software for which the source code is distributed along with the executable program, and which includes a license allowing anyone to modify and redistribute the software.

Source code is the actual instructions which programmers write to create a piece of software, the "recipe" for the program.

Once a program has been "compiled" into a form which can be installed and run on a computer, its source code is irretrievable.

It is practically impossible to make changes to a program without having a copy of its source code.

If a program's license includes the right to modify the program, this right is meaningless unless the source code is readily available.

Actual licenses for OSS vary between different companies and development projects, but they have certain characteristics in common:

In addition to its legal definition, another distinction between OSS and proprietary software is the way in which it is developed.

Proprietary software is created by a relatively small group of developers within a particular company, often working under deadline pressure.

They complete a program and then try to remove as many flaws (software errors or "bugs," and security "holes") as possible before the software goes to market.

Any flaws which remain after shipping time become the consumerØs problem, leading to lost work and frustration.

Purchasers of proprietary software become involuntary testers. What's more, if users find a flaw, even if they know how to solve it, the software license prohibits them from making the fix themselves.

Open source software, in contrast, is often developed by loosely organized communities of programming enthusiasts, collaborating via the Internet.

Anyone with an interest and some requisite degree of ability is welcome to contribute sections of the program or to look for errors in existing sections.

Because no one is excluded from the development process, potentially hundreds of people can contribute to a project, providing a diverse group of talents and techniques.

If a particular company has a financial interest in the success of an open-source project (through strengthening its brand name, increasing demand for related products, or through sales of technical support), they will often hire programmers to work on the project.

Other contributors (and for some projects, all contributors) may be individuals working in their spare time, out of interest rather than for compensation.

Open source software is economically viable and presents numerous opportunities for profit.

This seems counterintuitive, since the availability of source code allows the user to obtain a piece of software at no cost.

But many companies have already realized substantial profit from their OSS efforts.

Both individuals and businesses are willing to pay for this convenience. Additionally, the software itself retains some economic value even if it is given away for free.

The Open Source Initiative web site explains this way: "If having a program written is a net economic gain for a customer over not having it written, a programmer will get paid whether or not the program is going to be free after it's done."