A growler is typically understood to be a 64-ounce, reusable container employed to carry beer. Growlers are usually filled at the tap (from a keg) and are not factory sealed, meaning they have either screw-on lids or flip tops (as opposed to aluminum cans, which cannot be resealed once opened, or bottle tops which generally take special equipment to properly apply a seal).
The most common growler type is glass, either brown or clear, and has a screw-on lid. In some places, a smaller (often 32oz) growler is standard, and occasionally comes with the flip-top lid.
Since the standard glass growler became popular following its introduction in 1989, there has been some development of the style. While typical glass growlers have a small glass handle near the neck, some styles have metal or pewter handles, which vary in design and are sometimes customized to add identity to the growler.
There are a variety of alternatives to the standard, ranging from ceramic to metallic, offering a range of benefits and drawbacks, but all generally keeping within the central concept of providing an alternative to smaller, disposable containers, and giving the owner the opportunity to fill it from a draught.