The Open Source Software
OSS is the acronym for Open Source Software.
You can setup a suite of servers and tools for helping you in making websites and webprograms. The setup can consist of a webserver, a database server and a programming language - in this case the serverside script language called PHP, which again is an acronym, here for Hypertext Preprocessor.
The base is foundated in an operating system, that is also an open source system. Thanks to lots of dedicated people, around the globe, we can have this OS (Operating System), installed without a cost, and there are even huge amounts of different versions that have evolved from this OS named GNU/Linux.
The name Linux is from the founding father of the system, a Finnish man named Linus. So, what is GNU about then..