A glass was a container for serving liquid beverages for consumption, normally made of some type of glass, but metal or wood could also be used. Different styles were created from different cultures and for different types of beverages.
Glasses were typically replicated when an individual ordered a drink from a replicator. They were also used in bars to serve drinks from bottles. (TNG: "The Neutral Zone", "In Theory"; DS9: "Dramatis Personae")