Free software is an expression used to designate any computer program that can be run, copied, modified and redistributed by usersfree. Users have free access to the source code of the software and make changes as per your needs.
The philosophy of FSF - Free Software Foundation (Foundation for Free Software) takes into account freedom of expression and not profit. Therefore, a user who makes modifications to the program can give the whole community the possibility to benefit from the changes. But if you want to charge for your work, you have this alternative.
The FSF is a non-profit organization, founded in 1985 by Richard Stallman, considered the "father" of free software and creator of the GNU - Unix-like operating system. For Stallman, the Proprietary Software (those that are not free, such as the Windows OS or the Office Pack) are restrictive and unfair.
The freedom to use free software extends to any type of person or organization, on any system. computational, or in any type of work, it is not necessary to communicate the use to any entity specific.
Usage licenses were created to ensure equity of rights among users. The GPL - General Public License (General Use Public License) is the most widely used license.
Some examples of free software for personal use with a large distribution are: Linux (GNU/Linux operating system), The GIMP (image editor), Mozilla Firefox (web browser), among others.
See also the Meaning of Software it's the Meaning of Hardware.