There are loads of threads on the forum, I suggest you do some reading, then if you have any specific questions you can always ask and people will be happy to help. I even suggest going to google adwords learning centre, you can get a lot of info from there!
to make a long story short, it is advertising with an ad, and you pay for each time someone clicks on the ad.. it helps you get more traffic
http://becontext.com/pay-per-click-advertising/ http://youyield.com/ppc-advertising-networks/ wanna more details about ppc, go to google.com
obviously the biggest are ones most people have heard of such as google or microsoft, but for another easy-to use and inexpensive PPC advertising option check out pocketcents... 25 cents a click flat rate, local and industry relevant targeting