Open Source is a concept and license for publishing software along with it’s sourcecode. Under Open Source, software is licensed in a certain way which allows anyone to redistributable it at will and without cost. The source code is freely available, and it is modifiable by anyone sufficiently experienced in programming.
An example of a Open Source license is the GPL (General Public License). The GPL comes from the Free Software Foundation and is the main license of the GNU project which was Richard Stallman in the 1980′s.
The GNU project provides many of the key tools and components for what we call the Linux operating system. For example the C compiler which is used to build Linux is itself a free software project. Linux is licensed under the GPL.
The Free Software Foundation say that “Free software is a matter of liberty, not price. To understand the concept, you should think offree as in ree speech not as in free beer.”
Also they say “Free software is a matter of the users’ freedom to run, copy, distribute, study, change and improve the software.”
Another important project is the Open Source Initiative (OSI). They say “the basic idea behind open source is very simple: When programmers can read, redistribute, and modify the source code for a piece of software, the software evolves. People improve it, people adapt it, people fix bugs. And this can happen at a speed that, if one is used to the slow pace of conventional software development, seems astonishing.”