Open Source Software
Normal Software: All software (like the web browser you are currently using to read this) is written by programmers (also known as: coders; hackers; geeks) who are generally locked in dark rooms and flogged until they produce code for their evil bosses.
This code is what makes software work - but it is normally 'compiled' into a different form before being sold - this form is what you see (and is called executables; binaries; programs). The code is then locked in dark vaults that only the company can get at and modify… occasionally the company will release a new version (at a heinous cost) which fixes a few of the list of problems (bugs) that agitated customers have reported back.
Open Source Software: Open source is a fundamentally different process - the code produced by the programmers is not locked away but made freely avaliable.
This does many things: it lets the programmers talk much more to the users (no evil boss): it lets other people change the code to tweak it to do what they want it to (no locked vault); it lets bugs get fixed quicker; it allows more people to contibute to the development and hence more innovation…. I could go on (and probably will).
Lots of programmers develop Open Source in their own time - they are the hidden army of volunteers. Why? For a start - most programmers actually enjoy coding! However, most coding jobs don't get you anywhere near a final product, a user, or a network of peers - open sourcing allows coders to interact with peers and produce useful code - it feels much more like programming should do.
Variants
- FLOSS

