Your main character (or protagonist player) can have romances with certain NPCs according to his or her gender:

Female: Anomen
Male: Viciona, Jaheira, Aerie
Also if an NPC is having a romance with your character, they get various subplots:

Viciona: Attacked by a Drow and a Yochlol
Jaheira: Attacked by Bandits
Aerie: No subplot
Anomen: Seeks revenge for the death of his father.

Relationships develop according to your characters ability scores, alignment, or how you react to them talking to you. For example, Viciona likes a high strength ability, Jaheira likes honesty, and Aerie likes a Good alignment. Some romances always end in a bad way no matter what you say or do. For example, Viciona and Aerie sleep with you, then decide to break up afterwards. Viciona will sleep with you quickly, then starts treating you badly, then finally gives in to you once more. Romances happen when the NPC talks to you and you talk back very nice and if you have the things they like. After awhile, rest at an inn. You will see all the other characters, except you and the NPC you are having a romance with will sleep on mattresses. Then an animation begins, showing the two characters sleeping with each other. When the animation finishes, if it is a "bad ending romance" an argument will follow and the characters will break up. However, if it is "good ending romance", the NPC will keep talking to your character every so often. Some romances will not finish at the end of the game, but will continue into Baldur's Gate 2:The Throne of Bhaal.