Glass is the hard, transparent material that's used to make windows. While glass can be strong and sturdy, it's also brittle and can be broken easily by an errant baseball.
Glass has all kinds of practical uses, from car windshields to greenhouse windows to jam jars to drinking glasses. When someone offers you a glass of water, they're referring to the vessel itself as a glass and also to amount it can hold — the glassful of liquid. The Old English source of glass is glæs, ultimately from a Proto-Indo-European root that means "to shine."