Genre is a way of categorising texts according to similarities they share with those we already know. Texts that share content (e.g. chain saws, fondling couples), function (e.g. to frighten, to arouse), and form (e.g. books, films) are categorised and 'made safe'. But because, as they say, familiarity breeds contempt, genres can and do change – but slowly (see Pulp Fiction or Reservoir Dogs for evidence).
Genre is an important idea because it affects the production as well as the reception of texts. Writers know what we expect from a particular genre, and – to keep us receptive and comfortable (and hence – importantly for language study – more easily influenced or persuaded) – they will stay broadly within a particular genre’s expectations. Typical genres of fiction are adventure, detective and horror, and of non-fiction, reports (e.g. newspaper, school), biographical writing, advertising, recipes, etc. Taking account of genre allows you to comment on effective genre indicators and stylistic devices within a text. Texts may conform closely to the typical features of their genre, or challenge, mix and play with those genre expectations.