Definitions

Genre is essentially the way in which a text is put together (constructed). This can be important, for example, when distinguishing between a horror movie and a thriller, which can deal with similar subjectmatter, and look the same — lots of action set at night — but belong to separate genres (a horror film takes the audience into a supernatural place, where a thriller sticks to reality).
A media text is said to belong to a genre, as it adopts the codes and conventions of other texts in that genre, and lives up to the same expectations. Texts from different mediums may belong to the same genre (e.g. a tv programme like Dr Who and a comicbook like The Incredible Hulk can both be categorised as Science Fiction.)
By definition, all media texts are re-presentations of reality. This means that they are intentionally composed, lit, written, framed, cropped, captioned, branded, targeted and censored by their producers, and that they are entirely artificial versions of the reality we perceive around us. When studying the media it is vital to remember this - every media form, from a home video to a glossy magazine, is a representation of someone's concept of existence, codified into a series of signs and symbols which can be read by an audience. However, it is important to note that without the media, our perception of reality would be very limited, and that we, as an audience, need these artificial texts to mediate our view of the world, in other words we need the media to make sense of reality. Therefore representation is a fluid, two-way process: producers position a text somewhere in relation to reality and audiences assess a text on its relationship to reality.
A text is classified in a genre through the identification of key elements which occur in that text and in others of the same genre. These elements may be referred to as paradigms.
Audiences will expect a certain happening within the text by seeing what genre it is. If somebody wants to expect being scared and being thrilled they are not likely to buy a love story.