Growlers are typically sold and used for "take-out" beer. Many breweries will allow patrons to fill their own growlers in order to take beer home with them. Think of growlers as an adult doggie bag.
Growlers are generally made of glass and have either a screw-on cap or a hinged porcelain gasket cap, which can provide freshness for a week or more. A properly sealed growler will hold carbonation indefinitely and will store beer like any other sanitized bottle.