From The Web Hosting Show:

Web hosts go to a firm and buy a number of carbon credits to equal the amount of waste they are putting out. If they buy enough to cover their rear ends then they can claim to be carbon neutral. Think off it as a fat person who wants to get skinny, but instead of eating right and doing exercise they by “skinny credits” from Richard Simmons and they can claim to be skinny and health even though they haven’t changed a thing.