A drag queen is someone who theatrically performs hyperfemininity, and conversely, a drag king theatrically performs hypermasculinity.
Drag performance involves presenting oneself differently from one's day-to-day gender expression. This can involve costumes, makeup, and other artistic elements like singing and dancing.
While many drag queens are gay men, performing drag is not restricted to any specific gender or sexual orientation.
People do drag for different reasons, including to entertain an audience, because they enjoy taking on a performance persona, and to find community with other drag performers. Essentially, drag is a form of self-expression.