Open source is a relatively new name for a relatively old phenomenon that is also called “free software”. (this term preceded “open source” but it too came a long time after the phenomenon emerged). I have written a previous introduction to this phenomenon which I advise to read if you’re not familiar at all with the term.

To sum up, an open source program is a program that is accompanied with its original source code and can be freely used, modified, and re-distributed without any charge. It is possible to charge money for a package that contains free software components, but generally it is not economical to base a business on it, because of the fact that it can later on be freely distributed.

Some of the landmarks of the open source movement are the GCC-based software development kit, the Apache web-server and other Internet servers, the Mozilla project with its Firefox browser and other applications, various high-level languages such as Perl, Python, Tcl or Ruby; the X-Windows Graphics System and desktops built on top. There are also several operating systems whose every essential component is open source. They include the Linux (or GNU/Linux) Operating System, and some of the various BSDs

Open source software provides a low-cost, highly customisable, and often more reliable or technically superior alternative to commercial, non-free software. The open source world maintains an online web of mailing lists, web forums, IRC channels, web-sites and other resources with which people who are more knowledgeable in some respects help their less experienced peers.