The friend zone is when one person (and it's usually a guy) in a platonic relationship wishes to enter into a romantic relationship while the other does not.

"When a guy agrees to be friends, he's forced to stifle his attraction while regularly seeing and talking to the woman he's attracted to. She discusses her love life and has the audacity to ask his advice on it. He performs occasional "manly" household and automotive favors for the women. Essentially, he does everything a boyfriend would do – without the benefits." - Gina B., Chicago Tribune, 2007