What makes a community?

I don't know.

I've been asked this twice in different classes. I've come to the conclusion that a community is a group of people. That is the only thing that they have in common; from communities in your neighborhood or city, to threads and online games. They could have an interest in common, like: living in a particular city, playing the same game online, knowledge or interest in the same topic in a thread. But does that mean two people are a community? No -- maybe. It depends on one's definition of a community. League of Legends, a game a play online is supposedly a community. But many of them are trolls, and people who put other people down, or don't ever interact with other people in the "League community," can we say that it is a community then? Or do we just not count the people who don't contribute? Maybe a community also consists of people who contributes to the so-called "community." But either way, a community is whatever one deems it to be.