As with any theory, it is good to know this one even at GCSE, never mind A-Level. At A-Level, as always, you will need to be able to name it in your responses. Before we begin, we need to clarify what we mean by genre.
In its simplest terms, the genre of a media text is its type. Similar media texts can be grouped together in the same genre. Some genres are unique to one media form, while other genres can be found in lots of different forms. Horror is an example of a genre, and it can be found in various media like videogames, films, TV shows or podcasts.
When you look at any given genre, you will find lots of examples there. They will be similar in many ways, perhaps having similar mise-en-scène, soundtrack, storytelling etc. In the horror genre for example, we expect frightening scenes, blood, the threat of violence and unsettling locations. We expect characters to die horribly. In the third person MMO shooter genre, we expect weapons, player vs player combat, lots of arenas packed with powerups and character customisation with outfits and emotes.
Audiences like to know what to expect to some degree before they pay for a media work so that they know it will fit with their preferences. However, would you pay for the same horror film or MMO twice? Of course not. So for horror films, I might watch two that have a lot of things in common, but one is about werewolves and the other is about vampires. For my MMO, one might be set in a post-apocalyptic wasteland, but the other is set during the second world war. Steve Neale says that within any genre, there are examples of repetition and difference. Works within a genre repeat elements from one another because that's what fans of the genre are familiar with and want to have as part of those media texts. Those works also have some difference from one another so that consumers are encouraged to engage with (and pay for) lots of different horror films or MMOs (or whatever else) - not just one.
It's a delicate balance. If pieces of media within a genre repeat each other too often, people become bored of that genre. The market becomes saturated with too many similar products and consumers feel like they've seen/heard/read/played it all before. Worse, creators trying to make some fast money might try to release a poor quality example to 'cash in' on a popular trend, starting a 'race to the bottom.' If not managed carefully, this can cause entire genres to bubble - and what happens to bubbles eventually? They burst! The rhythm action music genre, for example, popular in the 2010s with titles like Guitar Hero or Rock Band, went from being a multi-million dollar genre to collapsing within a few short years. Fans, refusing to buy any more expensive plastic instruments or poorly made games that were rushed to market, got fed up of the amount of repetition.
Equally, if a media work does something very different or new for a genre, it runs a risk: it could be too different for fans of that genre and fail to engage them. Risky pieces of media often find it much harder to secure funding, especially in the videogame and film industries. Investors prefer creators to follow trends and repeat the genre as much as possible to guarantee that people will pay money for the product. Consider how clusters of similar films, TV shows or videogames often come out within a relatively short space of time. Media pieces that do do something different or new could also be the thing to reinvigorate the genre, becoming trendsetters within that genre. They might even start a whole new subgenre of their own. Fortnite, for example, could be considered to be the first of many games in the battle royale genre. If a piece takes a risk successfully, what will then happen is that later pieces in that genre will attempt to repeat that first piece's success. Think about how many similar MMOs launched after Fortnite, for example. It did something very new within its genre that lots of other games after it tried to repeat.
Eventually, of course, that new idea soon becomes repeated too often - and the genre needs to innovate once more. What this can mean is that within genres, certain things can come into (and go out of) fashion, changing genres over time. Some things, however, remain 'core' or 'fundamental' to that genre and stay the same over years or even decades. Neale also says that genres pay close attention to their context (i.e. what's happening in the world around them). Hollywood films often draw their villains from countries that the US is at war with, for example. Horror films often tap into current public fears or anxieties. Videogames do this very directly, especially in the sports genre with yearly releases of games like the FIFA series. They do this to align with what audiences are interested in right now and better guarantee their engagement. In Summary Steve Neale says that media products within a genre largely repeat one another's ideas and format (repetition). Each example within a genre tries to stand out from other examples (difference). If a media piece is very different to the rest of its genre, it might reinvigorate that genre if audiences receive it well. Neale also says that products within a genre will often be affected by what is happening in the world right now (context).