Free software, as defined by the Free Software Foundation, is software which can be used, copied, studied, modified and redistributed without restriction. Freedom from such restrictions is central to the concept of "free software", such that the opposite of free software is proprietary software, and not software which is sold for profit, such as commercial software. Free software may sometimes be known as libre software, FLOSS, or open source software. The usual way for software to be distributed as free software is for the software to be accompanied by a free software license, and the source code of the software to be made available.
To help distinguish libre (freedom) software from gratis (zero price) software, Richard Stallman, founder of the Free Software Movement, developed the following explanation: "Free software is a matter of liberty not price. To understand the concept, you should think of 'free' as in 'free speech', not as in 'free beer'". More specifically, free software means that computer users have the freedom to cooperate, and to control the software they use.
Most free software is distributed gratis online, or off-line for the marginal cost of distribution, but this is not required, and people may sell copies for any price. The capitalized term "Open Source" is attached to a definition originally created in 1998 from Debian's rewrite of the GNU definition of "Free Software". As a result, nearly all Open Source programs are Free Software, but there are some exceptions.
Although the open source and free software movements share almost identical license criteria and development practices, according to Stallman the respective philosophical values of the two movements are fundamentally different. Stallman endorses the terms Free/Libre/Open-Source Software ("FLOSS") and Free and Open Source Software ("F/OSS") to refer to "open source" and "free software" respectively, without necessarily choosing between or dividing the two camps, but he asks people to consider supporting the "free software" camp.
"Freeware" is software made available free of charge, but is generally proprietary, as users do not have the freedom to use, copy, study, modify or redistribute. Source code for freeware may or may not be published, and permission to distribute modified versions may or may not be granted, so freeware is gratis, and not libre software.
According to Stallman and the FSF, "free" software licenses grant:
the freedom to run the program for any purpose (called "freedom 0")
the freedom to study and modify the program ("freedom 1")
the freedom to copy the program 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")
Freedoms 1 and 3 require source code access, because studying and modifying software without source code is extremely difficult and highly inefficient compared to modifying annotated source code.
The Free Software Foundation web site provides a list of many free software licenses. The list is necessarily incomplete, because a license need not be known to the FSF in order to provide these freedoms.
"Proprietary software" is distributed under more restrictive software licenses. Copyright law and/or contract law restrict modification, duplication and redistribution by users; software released under a free software license rescinds most of these reserved rights.
The FSF free software definition disregards price. CDs containing free software such as GNU/Linux distributions are commonly for sale. However, since the CD buyer still has the free software freedoms, it is free software. Free beer software (freeware) which includes restrictions that confict with the FSF definition are considered proprietary. For example, source code may be unavailable, redistributors may be prohibited charging fees, etc.
Some people use "libre" to avoid the ambiguity of the word "free". However, these terms are mostly used within the free software movement and are slowly spreading.