Free Software

Free software is a matter of liberty, not price.  The Free Software Foundation (FSF), founded by Richard Stallman, has been a proponent of your freedom to use and modify software.  The FSF chose the Gnu as its mascot, mostly as a recursive acronym, explaining that "Gnu is Not Unix."  And the Gnu General Public License (GPL) is the definitive software license that guarantees your freedoms.  The Linux operating system and other great software is free for you to use, released under the GPL.  Even if Richard is a bit of an MIT hippie, the FSF and the GPL are really good things.

Open Source

There are also a number of "less free" software licenses, collectively referred to as "open source," which apply to software that has been released by its authors in source code form.  The licenses can be found at the Open Source Initiative.  Great software like the Tool Command Language (TCL - pronounced "tickle") are released under licenses like the Berkeley Software Distribution (BSD).