Linking is the currency of the internet. The more links you have pointing to your site, the more important your site is considered to Google and other search engines. Also, when you link to someone you are passing some of your credibility to them. But what if you won't want to do that? Use the nofollow tag.
If you put
define:NOFOLLOW into Google, you'll see this:
NoFollow: An attribute used in a link to instruct search engines not to follow the link. Correct syntax: <a href="http://www.domain.com/dir/page.php" rel=”nofollow”>Text Link</a>
Practically speaking, nofollow tells Google (and others) not to follow a particular link. You would use nofollow when you want to link to someone but you don't want to pass any "search engine goodness" to that person. Say you're writing a post about a notorious spammer and you want to link to him but you don't want the search engines to give him any credit for that link. Or, if you allow people to post comments on your website but you don't want link spammers to take advantage of you, then you could use a nofollow tag.
Two major websites, Wikipedia and Google-owned YouTube, recently started using the nofollow tag extensively; at least in part because many people were using their open content systems for link spam. That is, they were posting to the site for no other reason than to increase the number of incoming links to their own sites. That's not good SEO!
When you build links, be sure that you're doing it in a fair, accepted way. Don't just put a link to your site because you can, do it because it actually provides good value to the site you're posting to. Ask yourself, "If someone was doing this to my site, would I like it?" or my favorite "morals tester" of all time, "Would I want I explain what I'm doing to my grandmother?"
Remember, the web is just like the real world. Treat people with respect and you'll get along much better.