Open source software is software released under an open source license such as the GPL (GNU General Public License). Typically, anybody is able to view and rewrite the coding of open source software and anybody is able to redistribute the software (even for money), providing that it is released under the same license.
The term open source was first coined in 1998.
As stated in the preamble of the GPL, "The licenses for most software are designed to take away your freedom to share and change it. By contrast, the GNU General Public License is intended to guarantee your freedom to share and change free software--to make sure the software is free for all its users."
Open source software can evolve extremely quickly because anybody is allowed to modify it. Some examples of open source software are OpenOffice.org and Mozilla Firefox. The majority of Linux operating systems, the Linux kernel, and the bulk of software programmed for Linux is released under the GPL license.
Open source software is becoming increasingly popular as computer users become anxious of the hold commercial companies such as Microsoft have over them. Other people remain wary of open source software because they believe that it is inferior to commercial software or that it has increased security risks.