It’s pretty much self-explanatory. It’s when two people that get along great as friends but do not see each other as being “the one” decide to have sex with each other instead of (strangers) when the urge arises. They’d rather have it with a “friend” or someone they trust. FWBs are usually temporary until one of them finds a real mate.
A “booty call” on the other hand is someone a person meets strictly to have sex with. They generally do not bring them around family, friends, or co-workers. There usually is no serious introspective discussions about personal goals, dreams, or aspirations. In other words they are not seen as a “friend”.
Both FWBs and "Booty calls" are often kept secret or with few others knowing what is going on between them.