between vs among

• He sat in between Allen and Melissa. [=He sat in the middle of Allen and Melissa.]

• Mongolia is in between China and Russia. [=China is on one side of Mongolia and Russia is on the other side of Mongolia, and Mongolia is in the middle.]

• I will be at work between 8 am and 5 pm. [=8 am to 5 pm is the range of time you will be at work; you will be there for the time in the middle of those two times.]

• The train runs between New York and Boston. [=The train travels from one city to the other, and the two cities are stops at the ends of a route.]

Besides, some nouns are often followed by between: connection, difference, distinction, link, relationship

• Is there a connection between his arrival and her disappearance?

• There’s a strong link between clean drinking water and reduced infant mortality.

The most common verbs often followed by between are: choose, differentiate, distinguish, divide:

• [talking about judging a competition] I didn’t want to have to choose between the singers.

• Her money was divided between her son and her daughter.

• I sat among the other students. [=There were other students all around and I sat within the group.]

• There wasn’t much unity among the council members.

• He dropped the penny among the flowers. [=He dropped the penny somewhere in the flower bed.]

• Sidney has several dancers among her friends. [=Within Sidney's group of friends, several of them are dancers, but the rest are not dancers.]

• She wanted to be among friends.

• Among his books, we found some rare first editions.

• Paula always wanted to go swimming among dolphins.

In the phrases "among others" and "among other things", among means ‘as well as’:

• Her parents, among others, were worried about her travelling alone.

• Among other things, I still have to pack.