Vermouth is wine infused with herbs, alcohol, sugar, caramel and water according to specific recipes in Italy and France.
There are thee types of Vermouth:
Dry vermouth is white, usually 18 percent alcohol (36 proof) and contains at most 5 percent residual sugar. It's drunk as an apéritif and is a vital part of the dry Martini.
Sweet vermouth is white (bianco) or red (rosso) and is usually 15-16 percent alcohol (30-32 proof) with up to 15 percent sugar. It is also used as an apéritif as well as in slightly sweet cocktails such as the Manhattan.
Note: One of the major producers of vermouth is the company Martini, not to be confused with the Martini cocktail. Always ask your customer wether she/he means a Martini cocktail or a vermouth when she/he orders a Martini.