usually, between is used to refer to two people or objects. The dog ran between the two people. Among refers to more than two. The dog ran among the group of people.

It depends on the context. Sometimes they mean the same thing, such as when you are speaking metaphorically, "Among friends, you can speak more informally" = "Between friends, you can speak more informally." If you say you are sitting "among" your friends, this means sitting with your friends. However, if you say you are sitting "between" your friends, this means there are three of you and you are in the middle. Also, you might say "just between you and me" (which is the idea of something private between the two of you) but you couldn't say "just among you and me." I hope this helps!