Google adsense is probably one of the most common types of advertising, mainly because its so easy to use. I personally find it one of the best advertisers out there, and recommend it greatly. Adsense pays on a click basis, so your visitors have to actually click on the ads for you to generate income. Google will either mail you a check or transfer the money electronically once your balance reaches $100. Clicks vary in value, ranging from $0.01 to somewhere in the dollar range, and the prices are determined by the advertiser. Ads will automatically match your site content, but Google does not allow illegal or pornographic sites. Also, you can NEVER click on your own ads, because google will ban your account.
Adbrite may not be as popular as Adsense, but one benefit is that you choose when you are issued a check, anywhere from $5 to $100. Adbrite also pays on a per click basis, and is similar to Adsense. Adbrite is slower in sending a check, and you may have to wait up to two months before you receive your payment (net 60 rule), and this may cause people to reconsider. Personally, I think that this is the only shortcoming Adbrite has, and is easily ruled out by the fact that you don't have to wait until you have made a large sum of money before you are issued a check.
Bidvertiser caters more to the advertiser than the publisher, and pays on a per click basis, but brings into the game an element of competition. Advertisers bid on websites to place their ads, and if you have a large fan base, you could easily earn up to $0.50 a click. Payment is issued either through PayPal or by check once your balance hits a minimum of $10. An easy to use editing tool lets you customize your ads so that they match your site, and you can also block unwanted ads from your site. While not the best per click advertising site, it is definitely one of the good ones.
AzoogleAds pays on per impression basis, which is what sets it apart from other advertisers. Only consider using this advertiser if you have a large amount of visitors a day, but a low CPV (Click Per View) rate. With an easy to use interface and video ad capabilities as well, I would consider this advertiser before other CPI advertisers.
Don't discount independent advertisers. While maybe not as easy to use, you could probably work out a deal that fits your needs as well as the advertisers. Unfortunately, you might not get as much flexibility regarding price and placement, but don't disregard these entirely.