Basic Trope: A character who acts as a secondary Love Interest to one of the main characters in order to create tension and difficulty between two Love Interests.
Straight: Bob is in love with Alice, but Alice already has a boyfriend, Paul. Paul is from the outset not as suitable a match for Alice, but Alice is hesitant to leave Paul for Bob, providing drama. Inevitably she does choose Bob, or something happens to Paul.
Bob is in love with Alice, but Alice is married to Paul, an abusive, violent drunk. It is clear that Bob is a better match for her than Paul, but Alice refuses to leave Paul.
Alice and Paul are comically mismatched to a degree where it is patently obvious that their relationship is doomed to failure.
Alice and Paul were an item long before Bob entered the picture, and despite her feelings for Bob Alice feels that she cannot simply end her relationship with Paul so suddenly after all they've been through together.
Paul, like Bob, is a decent man and a reasonably good match for Alice, and she does care for him.
Alice doesn't really want to be with Paul in the first place, but her friends and family have sort of thrown the couple together and are pressuring her to remain with him.
Inverted: Paul, a minor character, is constantly trying to come between Alice and Bob, a Happily Married couple, because he sees himself as being a better match for Alice than Bob is.
Alice comes to realize that Paul is actually the better man, not Bob; she stays with Paul.
Alice sees Bob and Paul as equals, so there's no telling who she'll end up with, and no supplementary material that mentions Alice and Bob as the Official Couple.
Enforced: The writers want to create some romantic tension in order to prevent Bob and Alice hooking up too easily, and throw in a third character to make this happen.
Lampshaded: "It's like Paul just exists to get in between me and Alice!"
Invoked: Paul intentionally hooks up with Alice because he knows that Bob is secretly in love with her and wants to spite him.
Defied: Because Everyone Can See It between Alice and Bob, Paul decides not to involve himself with Alice.
Discussed: "Geez, I was sure that Bob and Alice were going to hook up before Paul showed up." "They may yet; I'm not sure how much longer Alice is going to put up with Paul's jerkiness. Plus, I hear he's been seeing Justine on the side."
Conversed: "Oh look, it's the character who just exists to delay the Official Couple getting together for a while. Let me know when she dumps him, will you?"
Alice is an immensely poor judge of character to stay for so long with Paul, a man who is so obviously a poor match for her; so much so that it's questionable that Bob is not in fact better off without her.
Paul is a decent man, and in trying to break up his and Alice's relationship for his own selfish desires Bob is actually the lesser man.
Paul is a decent man, but Alice just doesn't feel it with him. However, Paul is still in love with Alice, so, when Alice breaks it off with Paul to be with Bob, Paul becomes a broken man, and never finds true happiness ever again.
Alice is well aware that Paul is a terrible match for her, but feels hamstrung by her own feelings of loyalty, guilt and the pressures of others.
Bob realizes that Paul is a decent man and, not wishing to ruin his and Alice's happiness, withdraws from the contest for her heart to ensure her happiness. It is this act of nobility that convinces Alice that she would be happiest with Bob after all.
Played For Laughs: Paul is a gloriously stupid himbo who's so in love with himself that he barely notices Alice's existence. When Alice finally gets sick of him and dumps him for Bob, he's heartbroken... until he sees a gorgeous blonde walks past and immediately hits on her.
Played For Drama: Paul swears revenge on Alice and Bob after his rejection and begins to stalk them with violence in mind.