Courtesan is a generic name for characters found in both games. Your first encounter with courtesans in Baldur's Gate is likely to be with a pair who stand outside Feldepost's Inn in Beregost when you first approach the inn. After a brief conversation (or not), they will leave their initial positions and wander about the area. Although they do not initiate dialog, their audio greetings will play whenever you pass closely. These two courtesans play no role in the game except to provide a minute or two of entertainment. Which is probably very similar to, but much kinder than, what they have to say about you.
In Baldur's Gate II: Throne of Bhaal, you will find a few courtesans in the Tankard Tree in Saradush at night. They will ask you if you want to go on a date with them. Really though they are vampires in disguise who, rather than take you on a date, will take you to the Saradush old prison where a vampire ambush will be awaiting.