Between and among are prepositions. We use between to refer to two things which are clearly separated. We use among to talk about things which are not clearly separated because they are part of a group or crowd or mass of objects:

Typically, between refers to relationships that involve two people or things or to relationships in which every member is specifically mentioned. Among is typically used to refer to relationships of two or more people or things that aren’t individually mentioned or that are mentioned generally.

Most usage guides will say that you want to use between when referring to two people or things ('sitting between John and Carol') and among when referring to more than two people or things ('students chatting among themselves').