Anime genre labels are borrowed from magazine demographics, not from the stories themselves. That is why a “shonen” can be a cooking show and a “seinen” can be about a girl who runs a bakery. Once you know what the words originally meant, the shelf makes a lot more sense.
Shonen
Shonen literally means “boy”, and the term refers to the magazines the manga first ran in, aimed at readers roughly 12 to 18. In practice it describes a tone: clear goals, escalating stakes, a protagonist who improves through effort, and a cast that grows around them. Most of the biggest names you have heard of live here.
Seinen
Seinen targets adult men, and the shift is usually in pacing rather than content. Stories take longer to state their theme, violence has consequences that last more than an episode, and the protagonist is as likely to lose as to win. It is the label most often mistaken for “mature”, which it does not always mean.
A genre tag tells you who the publisher was selling to. It tells you almost nothing about whether you will like the show.
Isekai
The one genre on this list that actually describes a plot. Someone from our world ends up in another, usually one with game-like rules. The premise is a delivery system for wish fulfilment, satire, or both, and quality varies more here than anywhere else in the medium.
Slice of life
No overarching plot, low stakes, and a focus on small moments between characters. These shows are about atmosphere and company. If you have ever rewatched a sitcom for comfort rather than jokes, this is the same instinct.