There was a Wall Street Journal article on these a few years back. The bottom line: every airline has them, and if you have to ask, you're not eligible.

There can also be unofficial differentiation among people who are nominally in the same official tier. AA reportedly looks at your points/miles ratio (for non-AA types, you get more points per mile on higher paid fares, so this is an imperfect measure of revenue) in deciding whether to open up award seats in borderline situations and other such decisions for EXPs.