Free software
A category of software that allows the user to legally use, study, modify and redistribute the source code of a program. The term "free software" does not refer to price. Free software may be obtained for a fee, though it will likely be low, or free of charge especially when downloaded from the Internet. According to the definition given by the Free Software Foundation to be considered free software a program should offer users the "four freedoms":
The freedom to run the program, for any purpose (freedom 0).
The freedom to study how the program works, and adapt it to your needs (freedom 1).Access to the source code is a precondition for this.
The freedom to redistribute copies so you can help your neighbor (freedom 2).
The freedom to improve the program, and release your improvements to the public,
so that the whole community benefits (freedom 3). Access to the source code is a
precondition for this.
Most free software is licensed under the GNU GPL.