Those titles are based solely on the number of posts. Of course, the number of posts doesn't have to be related to one's experience, but at least you can see how long is the person hanging around here (or how much talkative he/she is
)
There are also some special titles, such as "guest star"
Oh, and I didn't intend to be a WME God, it's just a side-effect of having too many posts