Upper-crust means the same group as 'upper class' — the people at the very top of society — but it is an informal, half-joking way of saying it. You will meet it as a noun with 'the' ('she married into the upper crust') and as an adjective ('an upper-crust accent', 'upper-crust schools'). The picture inside it is a pie or a loaf of bread: the crust on top is the part that gets served first, or the part that looks best. Because of that image, the word is lighter and more teasing than 'upper class'. A history book will say 'the upper class'; a friend describing a posh wedding will say 'very upper-crust'. It is mostly British, quite dated in feel, and it usually carries a small smile — mild mockery of fancy manners, not real anger. Because of that tone, be careful using it about a real person to their face; 'upper-crust' said about someone is rarely a compliment. There is a story often repeated that the phrase comes from medieval families giving the burnt bottom of the loaf to servants and keeping the top crust for guests, but that tale is not reliable, so I would not teach it as fact.