Genres are an interesting thing because while they seem to define what qualifies as part of them, the reverse is also faithful to an extent. Genres are expanded in definition by the content that they contain, and in this way, the language surrounding them is flexible. A superhero movie from the '90s and a modern superhero movie will share necessary characteristics, but they will also tend to be very different regarding tone, pacing and other elements. As a genre's conventions are established and more content is added, creative alterations allow the way the genre is seen to be shaped through the years.
I first became interested in thinking specifically about this about half a year ago when I decided to play horror games to get over some fears. It largely worked, thank you. What I was most struck by though was the fact that the rules of the genre were so different than anything I had experienced.
Whereas action games tend to be loud and fast-paced, horror games were often almost silent except some quiet, moody background music and the occasional ambient sound in the distance. The pacing was slower and the plots deliberately crawled forward to build suspense. Within the very same medium, any given game in this one genre was likely to be radically different than anything I had played before, and for the first time in a long time, I was having fun getting lost.
What's odd is that I was never a stranger to the broader genre of horror in general. I had read plenty of scary stories and had seen a good deal of horror movies before I started playing their game counterparts, and yet, the interactive medium still felt almost entirely different. We have a tendency to call all action one genre, all comedy another genre, etc., and to a point this is accurate. However, the variety of media being used to convey the wider types allows for the creation of different sub-genres. A mystery novel is going to read differently than a mystery film will be viewed, and a mystery game will play out differently than either of those.
Of course, this is all assuming that genres stay in their enclosed spaces, but that's not usually true. Often, genres mix to create new genres. An action comedy will be different than a straight action movie or a straight comedy movie. I think it's this unlimited potential that has and will allow media to remain gripping to audiences in all stages of human development.