Members are automatically assigned a rank, depending on their number of posts. The minimum number of posts to earn each rank is:
New Member 20
Junior Member 50
Senior Member 250
Über Member 500
Fat Cat 1000
Postmaster General 2500
I have as much free time as a Wikipedia admin! 3500
Until now, only staff and those with at least 500 posts could give themselves titles to replace their ranks.
I am now allowing all full members (i.e. those with at least 100 posts) to give themselves titles to replace their ranks. These may be anything at all, subject to staff prerogative to remove anything obscene or otherwise unsuitable. Just go to your profile and edit it, placing your desired title in "custom title".