What's in a genre?

The speaker at our local writers' group writes historical fiction, but his characters have traveled in time so it's science fiction, and there's lots of action, maybe even war, so military fiction perhaps, aimed at adults, but some of the characters are teens so adult and young adult, but...

But what's in a genre?

I'm so behind with posting book reviews I'm wondering if I'll ever succeed in collecting them together by genre. Perhaps I should just post and click and post and click again. But let...