In terms of Adsense they render two type of ads (and no, you can't choose which type you get :) ~ pay per click and pay per impression. They're pretty much self explanatory; with pay per click you get revenue each time someone makes a legitimate click on an ad and with pay per impression you get about a Krona or less for every 1000 times the advert(s) appears. 

Pay per click easily generates the most money and you can expect to get about 1-3 clicks for every hundred or so visitors which means you need to be getting 1000s of daily visitors to make even a modest amount of money. It is worth it but can takes a few years to build up that level of traffic to reap the (again modest) rewards.