Open source software is software made free, usually by the architect, for all to use. The word free in this context implies free to be modified and redistributed.
Source code. The Source Code is the very basic aspect of every and any form of software.
Source code is a very important criteria in naming software as being open or closed. Software is considered open if its source code is put in a format which makes the code easily available and changeable.
http://www.opensource.org/docs/osd
Open source software has proven to be a relevent choice for many companies in today's world. Linux, an open source operating software is perhaps the biggest open source project. Linux was created by a student in Finland and by design made available his source code, which in turn allowed for many to contribute to the source code and eventually make Linux a viable operating system.
As the world grows closer and closer, ie, technologically, the application of open source also grows.
BATTLE WITH CLOSED SOURCE
This concept that open source presents seems attractive and one would assume that it would be the leading form of software available. However; open source has been and is still playing catch up with closed source software. Closed software is the opposite of open source; its source code is only viewable and revisable by its authors. Most software present today are closed source.