A fuck buddy is similar to a friend with benefits.
Person 2: She's not my girlfriend. We're just fuck buddies.
Strictly, for the term 'fuck buddy' to apply, both people involved have to be single.
Often used to describe two people who use each other solely for sexual gratification and nothing else. Usually this is a sexual relationship that still allows for each partner to seek sexual and emotional relationships with other people (hence, no attachments) but can rely on each other for a quickie or a booty call .
"Nope - we skip the serious and go for the fuck. We're fuck buddies."
Margeret: Oh, he's just my fuck buddy.