Mini:Open Source
"Open Source" means that the source code for software (such as a web browser or an operating system) is distributed under a free license (create) that allows everybody to use the code, modify it, and redistribute it.
[edit] GNU General Public License
Open Source programs usually come with a so-called GNU GPL License, which allows every user to modify the code and redistribute it. Open Source is the opposite of the so-called "proprietary" or "non-free" software, which comes with other licenses than the GPL. The GPL license has no limitation regarding the number of users per license, other than for example, the Microsoft Corporation, which charges license fees for every single computer their operating system runs on.
[edit] Other Definitions
Open Source does not only apply to software, but also to knowledge and information. The internet database Wikipedia is an example for the idea, also called Open Content.