If you go in to any brewpub where they brew their own beer, oftentimes on the drink menu you will see several sizes: pint, mug, pitcher, growler and keg (of various sizes). One of the most common questions people ask me regarding these vessels is "What's a growler?"
The dictionary defines a growler as: a pitcher, pail, or other container brought by a customer for beer.
Many brewpubs carry their own growlers printed with their logos, so you can show all your friends where you got their delicious beer.
The most common growlers are jugs made of clear glass, with a small handle and small mouth. Sizes can range anywhere from a half-gallon to a gallon.
The bartender should wash and sanitize each jug before filling. Once filled, they put on the cap, usually metal, and may or may not use some sort of tape to seal the growler air-tight.
Oftentimes their will be two separate prices for a growler of beer -- the price for the jug itself (a deposit) and the price for the beer to fill it. You will not have to pay a deposit for a growler if you reuse the same one over and over. It is best to thoroughly wash your growler with hot-soapy water before-hand, but the bars will sometimes also wash/sanitize your growler again for you before filling.
Remember to keep your beer cold at all times! And it is best to consume the whole growler within 72 hours (3 days). But depending on conditions, your beer may stay good for up to two weeks, if kept quite cold and sealed very tight.