Romance is a type of relationship where two characters fall in love over the events of the story. Sometimes the characters will be boyfriend and girlfriend when the story starts, and then progress deeper into their relationship or end up getting married, and sometimes they will start out not knowing each other, feeling neutral toward each other, or even hating each other. Romances can be an element in any type of fiction, not just Romantic Fiction. A romance can be placed in almost any genre of fiction. Sometimes the romance will play a big role in the story, and sometimes it will be just there for the author or maybe to appease the readers.
Gay and lesbian romances tend to be less common in fiction, but they can be there nevertheless.
When two characters are both in love with another (usually a girl and two boys), it is referred to as a Love Triangle.