A fuck buddy is when two people just want sex, no strings attatched. Usually people do this because they aren't getting sex elsewhere therefore feel the need to have a 'fuck buddy'. With the two people being fuck buddies they can sleep with or have sexual encounters/intercourse with a different person without being in a relationship.
Deirdrie: Margeret, whats going on between you and Boris?
Margeret: Oh, he's just my fuck buddy.