A boy is a male child. When a baby is born, the doctor or midwife often says, "It's a girl!" or "It's a boy!"
A boy usually grows up to be a man, just as a girl most often becomes a woman. A delivery boy is a young man whose job involves taking pizzas to people's homes, and you might call to your dog, "Here, boy!" unless your dog is female. Boy is also an exclamation: "Oh boy! I can't wait for my vacation next week!" The Old English root is boie, "boy," but also "servant" or "commoner."