You're not going out with him/her, but you do like them, and they like you too. Basically you act like you're in a relationship and get jealous when they talk to the opposite gender.
(In Kelly's head): Danny is so cute!! Aww we would look so cute together!!
Danny: Hey Kelly.
Kelly: OMGOMG ARHAERIHgshroihERK HI DANNY!
Danny: You cute. I'ma talk to Cindy.
Danny: Hey cindy, you hot.
Kelly: WTFFFF WE'RE IN A CRUSHATIONSHIP!! YOU BROKE MY HEART!
Danny: *just ignores* So yeah Cindy, Prom 201_?