As I interview more and more published authors, I am gaining a new vocabulary. A month ago, I had no idea all this things existed. This is shameful to admit because I have been published since I was in college (yes, college newspapers count!).
Two things have been hammered home to me over and over: 1) Stop arguing with the way things are. (I have been known to explode with "that's ridiculous!" and "why does Amazon make this so confusing?!" 2) Work with the system, not against. (This is the same point made two ways, probably.)
So, to explain:
Category: This is the word used to classify books usually at more of a macro level. Fiction is a category. So is Non-Fiction, Research, etc. The trick is that a category doesn't tell you anything about what is in a book. These can drill down further to subcategories like New Adult or Cooking or Poetry. Some resources, like the Library of Congress, will only use the word category to describe the most basic 'Fiction v Non-Fiction' bucket and call everything under that a 'sub-class'. (https://www.loc.gov/catdir/cpso/lcco/).
Genre: This is the word usually used to give the category of what the book is about. An author recently told me that she writes 'clean cozy' mysteries. This is a specific Amazon genre (Mystery) and (Subgenre). People know they will be getting a mystery book with little gore, no smut or NC-17 obscenity, and a little-town or tight community feel. As you can see from above, Amazon likes to drill down the genre to make it easier for buyers to find exactly what they want.
Amazon genre/subgenre classifications that all but spoil the ending of a book. To make it more fun, they call the entire category/genre/subgenre tree the "category". For example, this is the category for a romance novel that features Mermaids:
Keywords & Tags: First of all, "keywords" and "tags" are often used interchangeably. Basically, they are descriptors that let people know certain things are in your book. Amazon calls them "keywords" and Wattpad calls them "tags". Either way, they let people search for things beyond the category of your book. You often see them at the bottom of posts (like this one) or even Amazon reviews - sometimes with a hashtag to make it easier for search engines to find.
For example, if you want people to know that your mystery features a lesbian sleuth working out of Chicago then you may put "LGBTQ", "Lesbian", "WomanDetective", "Chicago", "Midwest", "StrongFemale" and "Urban" as tags/keywords.
I hope this helps. Write on and be well!
xoxo - Bella