Free Software (sometimes also referred to as Free and Open Source Software, FLOSS, FOSS, Software Libre, or Open Source) is software under a protective license guaranteeing that anyone can download, share, and -- significantly -- change it in any way they want, and redistribute the results. Practically speaking, you might never want to change the software yourself, or even have a resource person who can read the source code (the instructions written by programmers). But you can ask others to make changes for you.
The ability to change the software protects you in many ways:
- Free Software projects usually don't vanish, unlike non-free software from some companies that can no longer be licensed if the company goes bankrupt or decides to discontinue a specific product.
- The software can be used and customised free of charge.
- Nobody can suddenly take away features or change the terms under which you're allowed to use features.
- If an organisation wants a feature that the software doesn't provide, the organisation can just hire someone to create it. Of course, the organisation can also submit a feature request to the project team as with any software product.
- Similarly, anyone can fix a bug (error in the software) if he or she has the skills to do so. Because the source code is available, clients can also find bugs more easily.
- Members of the community have much more input into how the project develops, because they can understand the product by reading the code and can make changes. Furthermore, many people can try different implementations of features and the community can decide by vote or consensus which one to make official.
Nearly any software that qualifies as free also qualifies as Open Source, and vice versa. The main reason that two different terms exist is that "free software" emphasizes the freedom aspect (that you aren't under the control of the original programmers) whereas "open source software" emphasizes the convenience and potential for innovation provided by having the source code available.
When you install and use most Free Software applications you'll notice there's no annoying click-through software license imposing a thousand things that you can or cannot do with it. That's because free software doesn't limit your right to do with the software whatever you want. Free and open source software have licenses, but they're simpler than and quite different from proprietary software ('closed software') licenses.