Types of advertisingThere are five basic types of advertising.
First Contextual Advertising
This type of advertising is usually CPC (cost per click) and is text based. The better ones send spiders out to read the content of the pages the ads are placed on and will show targeted advertisement related to the content of the page.
The big players in this space are Adsense by Google, Yahoo Publisher Network. Some have claimed that Adsense works better for them than YPN, and some claim Adsense comes out on top. Here at DSL we have been using Adsense from day one, and I can tell you that it has done very nicely for us here. I have no complaints.
There is also another type of contextual advertising where the terms found in the content of the page become highlighted, and then when one mouses over the term, a small bubble window pops up with an advertisement. A major player in that space is IntelliTXT (vibrantmedia.com). This type of advertising seems to be popping up more and more, but its reception has been weak.
Currently, word on the street is that Google's is better at placing relevant ads on publishers sites than YPN, and that the pay off tends to be better.