What's the difference between 舅舅 and 叔叔?
Uncles of mother's side are called 舅舅, while uncles of father's side if he's older than your father you have to call him 伯, and 叔 for younger ones. If you have more than one 舅 or 叔 or 伯. You need to add some prefix to specified them, like 大舅 for the oldest brother of you mother, the following are 二舅, 三舅... For 叔叔 伯伯 they apply the same, except there is no 大叔, and the 叔's numbering should inherit from 伯s(e.g., 大伯，二伯，Your Father，then 三叔,四叔).  There also have a word 大叔, but it's just a term to refer to some random older man...

For 叔叔, 伯伯 they can also refer to your parents' friends of their generation or your friends parents who are of same generation of your parents. Usually you won't use the term 舅舅 for this kind of acquaintances, but it happens when the guy is your mother's distance relatives, they might come from a same village with a same family name.

EDIT: 叔叔 is also used as a term of address for men (unrelated to you) who are roughly your father's age.