There are fifty levels, with fifty being the highest. In the FAQ, they say 48 is the highest achieved - though they must mean as a working vocabulary. If you get three right in a row at one level, you go up a level (after 3-4 questions to establish your starting level). If you get one wrong, you drop a level. I average around 45, but have reached 50 (briefly). Most games I expect to reach 48, though I generally won't stay there very long.
The format is very simple: colobus is a type of monkey, for example, the word itself doesn't mean 'monkey' but that's the answer you have to choose. It's also an American site, so you may be given the American word for something and expected to know the English equivalent. A knowledge of medieval clothing, armour and weapons seems to help too.
All in all, well worth bookmarking and playing, to see if you're as smart as you like to think you are.