the link lepium posted will explain things.
Also check out the B-School Reviews forum:125
Ultra Elite are the "M7" schools, with some "sub-clustering" as shown below:
Elites are the next 9 schools, with Haas and Tuck starting to push the "M7" envelope in most rankings.
After that I'm not familiar with all the schools, but you get the gist.
As for extracurriculars, don't think of them as which ones are "high advantage". Do something you're passionate about that helps other people, and if you can display that passion in your essays, then your extracurriculars will be "advantageous". Someone can do high school soccer coaching and be truly passionate about it, which will come across a lot better than someone who just participates in Toastmasters on the side.
*edited by Pelihu - you forgot Darden and included both Duke & Fuqua in the Elites list*
GMAT Club Knowledge Vault: