Understanding film genre

Genre is a type of story, for example:  thriller, comedy, science fiction. Films within a particular genre share things in common. A science fiction film usually contains some story line where the science of that particular time undertakes something out of the norm, like contact with alien life forms or time travel. The thriller genre is different from most other genres because on the surface many of them look entirely different from each other. Thrillers generally have similarities though, for example they all involve some sort of dramatic plot twist, like in the Shining where Jack Torrance goes mad and starts to attack his family or when the shark in Jaws attacks the defenceless civilians.

Here are a few examples of typical thriller posters.