The margarita glass is used primarily for serving Margaritas, though they can also be served in a cocktail glass. The double-bowl shape of the margarita glass is a fun and distinctive shape that works particularly well for frozen margaritas and the wide rim makes it easy to add a salt or sugar rim.
Margarita glasses can come in a variety of sizes and can range anywhere from 6 to 20 ounces. The smaller glasses (right of the photo) are nice for drinks with no ice, the medium glasses (middle) are good for frozen drinks, and the large bowls (left) are good for drinks served with either a lot of ice or large frozen drinks.
Some margarita bowls can get to ridiculous sizes, topping off at 60 ounces. These would only be good as a novelty to share with a table-full of friends, each with your own straw, of course.