Computer programs in which the source code is available to the general public for use and/or modification from its original design free of charge, ie, open. Open source code is typically created as a collaborative effort in which programmers improve upon the code and share the changes within the community.

The underlying programming code is available to the users so that they may read it, make changes to it, and build new versions of the software incorporating their changes. Anyone with programming experience can revise and change the programming code to suit their individual needs.

Often you will pay for someon's knowledge and sometimes you will pay for their work when they customise one of these open source products for you.