I'll sum it up for you (or try, anyway!)
There are Junior Members, Members, Senior Members first. These make up the vast majority of XDAs userbase.
Recognised Contributors are nominated by other users and/or Mods/Developers. They are recognised for their contributions which are not specifically Development, such as detailed guides, one-click utilities and other tools/mods.
Recognised Themers are also nominated by users/Mods/Developers, for their work themeing custom ROMs and providing standalone themes for flashing.
Recognised Developers are nominated and approved by the Developer Committee, for their work in Development whether it be ROMs, Root methods, bootloader unlocks etc.
Elite Recognised Developers are again assessed by the Developer Committee, and are deemed to have gone above-and-beyond in providing knowledge/hacks/tools/apps/ROMs for the XDA community.
Forum Moderators are appointed by the Moderator Committee, after submitting an application (see the 'sticky' at the top of this forum - currently applications are not open). They are the first-line of keeping the forums tidy, friendly and orderly.
Senior Moderators are the next step up the ladder, providing site-wide support for Forum Mods and looking after the nitty-gritty issues, as well as areas of XDA not policed by FSMs, such as the Marketplace.
Administrators are next. There are several Admins - each look after a specific part of XDA such as the Portal, the Members and the User Experience of XDA as a whole.
At the top is Mike Channon, who is overall Forum Administrator.
Other titles you may come across:
Moderator Committee - sits on the Mod Committee
Developer Committee - as above, but for Developer Committee
XDA Newswriter - publishes articles for the Portal (front page)
Retired xxxx - have since officially 'left' their XDA position
Hopefully that clears most of them up for you
Contrary to what a lot of people expect, the thanks-meter does not figure at all in determining a users tag.