Key Concepts
Genre does not rely simply on what's in a media text but also on the way it is put together (constructed). This can be important, for example, when distinguishing between a horror movie and a thriller, which can deal with similar subject matter, 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.)
E.g. women's magazines always present an attractive model on the front cover. Media texts follow sets of conventions in the way that they are constructed. You see a contents page in a magazine before any feature articles. In movies, a romantic comedy always ends with a wedding. Often content and style are closely interlinked.
No — genres are described as dynamic, i.e. the boundaries are constantly changing. Individual texts can challenge conventions, and defy certain parts of the usual genre categorisation — for instance, recent movies such as Superbad, Forgetting Sarah Marshall and Knocked Up (all from the same producer, Judd Apatow) have redefined the romantic comedy genre, making the humour cruder, and telling stories from a male character's, rather than a female's perspective. Genre texts would get very boring and predictable if they all followed exactly the same conventions — no audiences would want to consume new ones, they would just keep on revisiting old favourites.