Open Source Software is primarily defined as software which is freely redistributable and includes the source code. The licenses under which OSS is released vary greatly, but these these two points remain consistent. The complete Open Source Definition can be found here. This is vastly different from the mainstream software industry where source code is highly guarded and programs are only distributed in their binary, unmodifiable format.

The most important aspect of the open source movement is the participation of users. When a user or users what a feature or bug fix for a program, they have traditionally been at the mercy of the software vendor. However, with open source they can modify the program to their own needs or fix what is broken. Many users will help develop the program for free, simply to improve the product and benefit the community.