@Ryan Marcus French
You really think that this helps , i need to know what it is now that are alot of sources that are invalid , please dont take the piss.
Actually, it should help. The first link is Wikipedia, which gives you a quick, top-level explanation. The next couple are FlipCode, which is a good site for game code info, and some StackOverflow Q&As which is a great resource to use for any code. There's an MSDN article from Microsoft, the Chris Hecker article zbethel mentions, etc, etc, etc. A basic concept like perspective correct texture mapping is not something that will change over time, so these links have not nor will they somehow become "invalid".
So, yeah, honestly, and with no mean intent, there's a lot more there than you'd get in a forum post.
No taking (or giving?) of piss intended...