Levels, or classes, are the representations of how much Steem Power (the background currency is vests, actually) )an account is holding.

Once there was a page at steemd.com showing these levels and amount necessary to reach them along with the number of accounts.

You can find the image of these levels in this article. The names in the image are official while the names I am referring to are common names used on the platform. You know, marine fauna :)