Carboys are large jug-shaped containers typically made of glass or plastic. They are used in brewing for small batch fermentation. Carboys usually range in size anywhere from ½ gallon to 6 ½ gallons. An air lock and stopper or rubberized bung are typically placed at the top of the carboy to create a seal that allows CO2 to escape from the fermenting beer, while still maintaining a sanitary environment inside the carboy. Glass carboys are airtight, which can be better than air permeable plastic carboys, but glass carboys are also more fragile and dangerous to work with. Additionally, glass carboys are also less susceptible to internal scratching since their surface is much harder than that of a plastic carboy.
Below is a photo of a carboy filled with fermenting beer.