While having Googlebot index your site more quickly is almost always a good thing, there are times when you don't want certain pages or images indexed. Most "reputable" spiders will obey a directive given by the robots.txt file. This file is document that tells spiders what they may and may not index. You can also explicitly instruct a robot not to follow any of the links on a page by the following meta tag:META NAME="Googlebot" CONTENT="nofollow".
Because of how these bots work and the importance they place on text links, many people have begun placing keyword filled text links to their website in their signatures on blogs and other comment sections. To reduce the impact that these have, you can instruct spiders not to follow one specific link by placing the following in the anchor tag:rel="nofollow". This will reduce the outgoing number of links and help you to maintain your pagerank.