How to solve nine out of ten Steam problems!

[Varsity]

You know what the real problem with Steam is? No, not that - or that! The real problem is that there are so many things to go wrong, and so few of them that can be resolved by anyone but the user. Between the Steam client sitting on your computer and the Steam servers sitting on Valve's 120-odd computers, there are a raft of hardware, software and connection errors to dodge.

http://varsity_uk.blogspot.com/2004/12/swap-out.html

The article explains what, where and to a certain extent why problems arise and how to go about isolating them in order to fix them.