Compare and contrast an in-group and an out-group.
These terms can only be truly defined with respect to any given person. Sociologists use these terms to refer to groups that any given individual identifies with or does not identify with. In other words, these are subjective terms because what is an in-group to one person may well be an out-group to another.
For a given high school student, then, the "jock" clique might be an in-group. The student might identify them by the fact that they all wear their letter jackets, or other clothing that has to do with the sport they play, to school. The student and other members of that clique will see themselves as superior to some other group. An out-group to that student might, for example, be the members of the theater club. The jock might think of those students as a bunch of geeks who are beneath their notice.
So, we can only define in-groups and out-groups in terms of our own attitudes. I think that you should try to identify a group that you identify with and compare and contrast it to one that you think of as very different from your group. This can be anything from a high school clique to a religion to your nationality or ethnic group.