A muddler is a bartender's tool, used like a pestle to mash or muddle fruits, herbs and spices in the bottom of a glass to release their flavor. Muddlers come in a variety of materials and most are about the same size and basic design. They are typically 6-8 inches long and about 1-2 inches in diameter, the perfect size to reach the bottom of the average mixing glass or cocktail shaker.

Wood muddlers are the most popular and have been used in the bar since the first drinks were mixed. They come in two basic shapes: one looks like a miniature baseball bat with a flat bottom and the other is rather straight, similar in design to a wind chime or hand bell. These are favorites for bartenders because they are durable, get the job done quickly, and are useful for other things (like cracking ice cubes into smaller pieces).

In recent years, there has been a revolution in muddler design and many are now made out of plastic, stainless steel, or a combination of synthetic materials. It is also more common to find a muddler with teeth on the bottom. These are used to mash ingredients more efficiently and are great for quickly extracting juice from berries and other produce.

Cocktails that require the use of a muddler include:

Mojito made with light rum

Caipirinha made with cachaça

Caipiroska made with vodka

Mint julep made with Bourbon whiskey

Old Fashioned made with whiskey or brandy

More Info: en.wikipedia.org