Much like how animals are classified by Kingdom, Phylum, Class, Order, Family, Genus and Species, shoes can be classified into various categories and sub-categories.
Let's take, for example, flip flops.
Flip flops are shoes, of the sub-category "open-toed shoes" and perhaps also of the sub-category "sandals". But they are NOT mandals. This is an important distinction. They come in many shapes and sizes, made of various materials and can be worn by men (see aforementioned distinction about mandals), women, children and everything in between.
Once you get to the sub-category "flip flops", you are likely looking at a shoe that consists of a piece of material between the big toe and the second toe. Beyond that, there are several options: ankle strap, heel strap, high heel, flat, etc. And then there are multiple materials: leather, pleather*, sequins, other shiny things, cheap plastic foam, etc.
Now, generally, the type of materials used and general look and style of the flip flop determines the types of occasions for which it'd be appropriate. A leather-and-sequin-strapped shoe with a slim profile and stacked heel is probably appropriate for many summer occasions. Depending on where you live and the circumstances involved, I'd allow that these shoes would work for attending a bridal shower, baby shower, rehearsal dinner, birthday party, date night or a day at the office.
However, footwear made of plastic-esque foam that may or may not squeak when you touch it is an entirely different story. This particular sub-segment of shoes can be further classified beyond "flip flops" to be described as "shower shoes". This moniker comes from the fact that these shoes are water-resistant and made of such lab-created materials as to resist damage from water, should they be worn in the shower. You know, like when you're camping - or in college - and share showers with a bunch of creepers/strangers/dirty roommates and don't want your bare feet to touch the tile. That's when you might want shower shoes.
Some people make the mistake of thinking "shower shoes" can be worn in public. This is a mistake. If they were meant to be worn outside of the shower, we would not all refer to them as "shower shoes". Now, before you get all riled up, there are a few exceptions to the "not in public" policy, as it were. Approved locations for these shoes: situations that immediately precede and/or follow a shower, e.g. wearing home from a soccer game, after removing your muddy cleats and socks or while walking to/from the beach (ocean/lake) or pool.
Note: in the approved locations I did NOT list the office, the mall, a wedding, a bat mitzvah, or any other occasion that calls for make-up.
I know that some people have trouble understanding that not at all flip flops are shower shoes (even though, generally, all shower shoes can fit into the sub-category of flip flops). I hope this helps.
*pleather = faux leather. In Seattle, we call this "vegan".