If you want to understand online advertising or simply want to understand who does what in the world of online advertising, then read the following three blog posts. A very easy to read and comprehensive guide to online advertising, relevant to Google adwords, Live Adcenter and Yahoo Advertising, and probably a load of others that exist.

Online Advertising Business 101, Part I - The Online Advertising Value Chain

Online Advertising Business 101, Part II - How does adserving actually work?

Online Ad Business 101, Part III - Ad Networks

Now having read these posts, it's time to start an online advertising campaign, or get someone who knows what they are doing to run your Pay Per Click campaign for you!