I personally like the distinction made by Paul Fussell's book Class (note these are 1980s values):
Top Out of Sight - Billionaires and multi-millionaires. The people so wealthy they can afford exclusive levels of privacy. They live however they want.
Upper Class - Millionaires, inherited wealth. Those who don't have to work. They refer to tuxes as "dinner jackets." Typically "old money". Usually anglophiles, quiet with their money
Upper Middle - Wealthy surgeons and lawyers, etc. Professionals who couldn't be described as middle class. "new money" lifestyle. Usually own additional homes, flashy cars, may vacation in the Hamptons, sail, etc.
Middle Class - The great American majority, sort of. Secretaries, Accountants, Paralegals, Nurses, Teachers, Engineers, etc.
High Proletarian (or "prole") - Skilled workers but manual labor. Electricians, plumbers, etc.
Middle Prole - Unskilled manual labor. Waitresses, painters.
Low Prole - Non-skilled of a lower level than mid prole. Highly supervised, McDonald's and the like.
Destitute - Working and non-working poor.
Bottom Out of Sight - Street people, the most destitute in society. "Out of sight" because they have no voice, influence or voter impact. (They don't vote.)
The whole book is devoted to the subject, but he does break it down pretty well, admittedly with a 1980s style and bit of a glib approach. What I find most interesting is the mentality of each class and how much you can pick that up when talking to others today. He breaks this pretty squarely apart from income.
It's a great read, but what I particularly like is how cheap it is to affect an upper-class lifestyle just by avoiding the often costly middle and even upper-middle mentality.