Your title is a combination of quality/quantity of posts. You get or lose the title by being rated up or down.
Note that when you read a post, and are a registered user, there are arrows. If you feel the post was very helpful, informative, humorous/entertaining, or are hoping for oral, you may rate up the poster.
If the post was wrong information, stupid, insulting, or you have a headache you may rate the poster down.
You may even hunt down other posts the person has made and ruin/raise karma. You may have friends/enemies do it for you.
Get over 10k posts and you often get your own special title.
You can read the official answer on the FAQ section of the site.
Edited, Jan 2nd 2009 12:54pm by Jonwin