Chocolate syrup is a type of simple syrup flavored with either chocolate or cocoa powder, other flavors (typically malt or vanilla) and typically contains emulsifiers to prevent fat separation. Chocolate syrup is commonly homemade, but commercial brands are common. Hershey's makes probably the best known brand of chocolate syrup in the United States.
While chocolate syrup is most often stirred into milk, coffee, or poured over ice cream, it does have some uses in cocktails, where it provides sweetness, chocolate flavor, and texture. It does overpower drinks quickly, and its dark brown color can be offputting, so crème de cacao can be a good substitute.