Google search blurb: how does google decide what text to use?

That 2 lines of text that gives you a preview of the page. How does google know what to use? Sometimes it's not even on the page itself.

 

Google call it a snippet - if you want to change it, use a description meta-tag.
If you have a dmoz entry they'll use that, though you can use a meta-tagto over-ride that.
Otherwise it's another one of google's mysterious algorithms

http://www.google.com/support/webmasters/bin/answer.py?answer=35264&query=snippet&topic=&type=

 

<META NAME="Description" CONTENT="what you want google to show on the search goes here">