Free software, in a pinch, is software that users have the freedom to examine, modify, and redistribute.

The FreeSoftwareFoundation provides a more canonical Free Software Definition that you should read at some point.

Some people argue that the FSF's defintion is too rigid, and a more pragmatic view should be taken. The Open Source Initiative uses the umbrella term "OpenSource". Obviously, it follows that not all OpenSource software is also FreeSoftware.

Please be sure to know the difference and not to confuse the terms -- if for no other reason than to avoid inciting a flamewar.