They are titles or descriptions that are assigned to both reputation and post counts. I think lists have been posted before (forum search) but I have never officially posted them.
The Admins manage the text of those Titles and Descriptions.
Some info:
When you gain the Custom Tittle, you no longer are shown a rank underneath your name. You can put anything you want. See mine, for example?
be helpful and kind and you might get one yourself
Not really, if an admin notices that you have +75 reputation points or higher, they will give you a custom title and raise reputation points from the current value to +125.
Ghan has said he has fixed this. Monovertex please confirm this fix. This was only a problem with people that had signatures in the upper levels like not the special members but the respected members.