What is d20?
D20 is a rewrite of the 2nd edition Dungeons & Dragons game rules, what made D20 unique was that the core rules of this new Dungeons and Dragons system (D20 itself) is covered by the Open Gaming Licence, this means that for example we can use it to run this game without fear of being sued :-) and also other developers can write games around a known rule set that people like (we some do some don't, hey ho).
Since we are using D20 and the Core rules for this Chat, below is a link to the Open Gaming Licence, please do read it if you wish to know more, don't understand or are that bored at work :-)
Also on the Open Gaming Foundation site is the Core rules of D20 covered by the OGL.