During the homebrewing process, a primary carboy is used for fermentation. Once primary fermentation is complete, the beer is either transferred to a secondary carboy for conditioning or it can be transferred directly to bottles for conditioning. (This process of transferring is usually called racking.)
Polypropylene carboys are also commonly used in laboratories to transfer purified water. They are typically filled at the top and have a spigot at the bottom for dispensing.
Carboys come in various volumes ranging from 1 gallon to 6.5 gallons (4 to 25 litres). The term carboy used on its own will generally refer to a 5 gallon (22.5 litre) carboy, unless otherwise noted. A one gallon carboy is usually called a jug. A 15 gallon carboy is often called a demijohn (in the Philippines, "dama juana"). "Demijohn" is an old word that formerly referred to any glass vessel with a large body and small neck, enclosed in wickerwork. The word is said to derive from the name of a Persian town, Damaghan, but this is not supported by any historical evidence. According to The Oxford English Dictionary the word comes from the French dame-jeanne, literally "Lady Jane", as a popular appellation. This is in accordance with the historical evidence at present known, since the word occurred initially in France in the 17th century, and no earlier trace of it has been found elsewhere.
In Britain, demijohn refers to a one-gallon glass brewing vessel.
In Southeast U.S. slang, a demijohn jug, of any size, is referered to as a jimmyjohn.
The name is sometimes misspelled as "carboil" or "carble."