Primarily from ads, usually AdSense though other ad networks exist such as Yahoo Ads, Bidvertiser, AdBrite, ValueClick and others. Revenues are generated both for each impression (or display), but higher revenues are due to clicks, usually denoted CPM or clicks per thousand (where M is the Roman numeral 1,000).

Next is affiliate traffic, which is usually a CPA arrangement or cost per acquisition: every customer who clicks an ad on your site, and signs up for a service or buys a product entitles you to a revenue. Other affiliates arrange on a recurring percentage of sale, usually applied to monthly subscriptions (such as web hosting) or continuous pay (such as online poker).

A little less common is getting paid to do a review, though this works if you have a good online reputation or presence, or if you happen to be part of a blogging network.

boybi has a similar question; are the tsikoteers gonna be bloggers now
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BTW, you might want to join a blog network, which is basically an association of bloggers. These then do some form of revenue sharing by splitting ad revenues generated via referrals. Essentially, bloggers in a blog network have blog rolls pointing to other members' blogs. A viewer in one blog (say about cameras) might get interested in a blog in the blogroll pertaining to printers. He visits and thus generates revenue which is then split between the two bloggers.