Replicators house mini-transporters which are used to provide mostly food to the crews of Starfleet vessels and facilities. They must be connected to a power source and maintained with raw materials to keep them online. Any type of food or drink can be ordered, as long as the request is in the database. A common ingredient in alcoholic drinks is "synthehol." Synthehol is designed to give replicated alcoholic drinks the same taste, but without the disabling effects.
Certain items cannot be replicated. Living organisms cannot be replicated because of their dense and complex molecular structure.
On U.S.S. Voyager, replicator provisions were rationed to save power reserves.