When it comes to the difference between a mistress and a girlfriend, there are a few key things to keep in mind. First and foremost, a mistress is typically someone who is involved with someone else's husband or boyfriend - she is the other woman, so to speak. A girlfriend, on the other hand, is usually just referring to a woman that someone is dating - she may not necessarily be sleeping with him. Additionally, mistresses are often kept hidden from the public eye whereas girlfriends are usually more out in the open. Finally, mistresses are often looked upon as being nothing more than sexual objects whereas girlfriends are typically seen as being more than that.