Open Source Software (OSS) is software that is available free of charge. Not only is the software free, the source code for the software is also made available and should you so wish to do so, you may download the source code and modify the software to suit you needs. You are obliged to make the modifications you make public too.

A formal definition of OSS can be found at the Open Source Initiative.

This site is mainly for the end users of OSS and hence all you need know is that it is free and you may use it for your business or at home.