ya ,i use google adsense .its the best.
you get paid when visitors clicks on ads .the payment is variable and it depends on keyword bidding.
see google adsense in action here. http://www.outshineworld.com/forums.html
and if you wanna aply for it ,visit www.google.com/adsense
1) when people click on ads with ill intent
2) when the search engines knew this and do nothing about it, but they say the "protect you"... but then don't