Free Software is software that is more than "gratis", more than just Freeware, it's free as in "free speech". The beautiful thing about this kind of software is that its source code is freely distributable, can be used, changed and even sold by everyone, under the condition that the new software is also Free Software. Free Software is about sharing your work so that everyone can use it to its full extent, and, eventually, make the world a Better Place. If you have written some superdeluxe shareware program, at least consider making it Free Software. It'll make you feel better ;)
The guys that invented the so-called GPL license for Free Software are from the GNU Software Foundation; you can read more about Free Software on their website.
Unfortunately, many companies (including Microsoft) feel menaced by Free and Open Source Software. They want to protect their inventions more and more, so much that they even want to patent software ideas they did (or claim) to invent. This way, you could never create a Free piece of software that does something they patented. The well-known professor Donald Knuth writes it this way:
"Congress wisely decided long ago that mathematical things cannot be patented. Surely nobody could apply mathematics if it were necessary to pay a license fee whenever the theorem of Pythagoras is employed. The basic algorithmic ideas that people are now rushing to patent are so fundamental, the result threatens to be like what would happen if we allowed authors to have patents on individual words and concepts. Novelists or journalists would be unable to write stories unless their publishers had permission from the owners of the words. Algorithms are exactly as basic to software as words are to writers, because they are the fundamental building blocks needed to make interesting products. What would happen if individual lawyers could patent their methods of defense, or if Supreme Court justices could patent their precedents?"
Letter from D. Knuth
Now, you could choose not to make your software Free, but preventing others from making Free Software just seems Wrong, doesn't it? A lot of lobbying is done to pass laws that allow software patents (in the USA this resulted in the horrible DCMA - digital copyright millenium act), so if you can help in any way to stop this (for example, sign the eurolinux petition), please do so!
Thanks for hearing me out,