NSA can happen with virtually anyone, whether it's a random hook-up you met at a club or someone you met on Tinder.You don't need to have known each other that well before - or even after - you have sex.
You can hang out, watch movies, go on nights out with other friends - basically it's like any other friendship but with the added benefit of sex.
FWB can last for any number of years, often being on and off again type agreements.
Because you are already friends you normally go back to being just that once you stop having sex, although sometimes people struggle to handle this after getting too attached.
The terms you find differ on every site and app, with the user demographic usually influencing what dating terms you see.
For instance, sites that focus on romance and relationships tend to have dating terms that differ greatly from those found in hook-up sites.