They are 'content link' ads. If you mouse over one it will show an ad based on the word. It's a form of context sensitive ads. The ads (in theory) relate to the pages content.
Kontera
I am not sure on this company but most advertisers bid on words to show thier ad, the website is paid per click or view or sometimes only if things are signed up for it varies between ad companies. I believe google also has a version of the same concept along with several others.
Im not the webmaster but am a webmaster lol.