Bots like Google and Yahoo Slurp are the search engines indexing your site. If you block them, you will never get a listing and nobody will be able to find your site ever, except close friends and family, that is... The bots to block are the content scrapers and other hotlinking parasites that come calling. The key is learning which ones are good and which ones are not... for the most part you want to enable all the big ones like google, yahoo, and msn and screw the rest of them unless they support your niche, at least that's my take on it.