Linkshare is an affiliate management broker where you can find advertisers who PAY BY COMMISSION from a predefined set of action -- per sale, per lead or per signup. No matter how many times the ads are viewed or clicked, Linkshare only pay when the action is done. Plus, you have the option of selecting advertisers you want shown in your site

Adsense is a contextual advertising website where they automatically match the ads based on the content of your site. You can earn by

1. Pay per click - this is the common way to earn which is you earn every time a visitor clicks on your ads
2. Pay per impression - an advertiser can specifically target your site and show ads where you earn via cost per 1,000 impression (e.g. you earn every time someone sees your ad, even though they don't click on it)

You can use both in your website to maximize revenue potential of your website. Or experiment and check which ones give you the most bang for your buck