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People install a toolbar from usually a gaming site. When they search for something an advertiser has targeted an ad pops up.
Advertisers bid on keywords and URLs.
You can rape my "thanks" button 
go here and install this.... http://gamevance.com/ thats Trafficvances toolbar...
or
http://hotbar.com/ for lead impact.......
you will need to be on USA proxy to see pops.....
then you can get an idea of the whole process
If you direct link with PPV you do not have any ad creative - the offer landing page is loaded in the popup.
Alternative is to create your own "Landing Page" which will be loaded in the popup and then link to the offer.
Budgets vary greatly depending on offer, offer payout, targets, and whether you use a landing page or not. So no hard and fast rules on that one.
But - check out the following along by Liane which has a great discussion going on about budgets and spend per target.
Cheers
Tijn
You'll have to test that one, it depends on many factors.