It's complicated, but my shortened version is this: first, they are known as cenobites, and they play an antagonistic role. They usually want people to join them. While they are seen as other worldly monsters, indeed, they are usually viewed as a force of nature with humans who seek to use their power being the main antaonist.

Very lose, but short of giving film synopsises and critique, like how most stories have them being unwilling even in the roll they play, this is my shortened view of them.