Sweet vermouth is an aromatized, slightly fortified red wine with a moderate sweetness balanced by a bitter aspect. The word Vermouth comes from the German "wermuth", or wormwood. While there are many styles of sweet vermouth, and no recipe is standard, sweet vermouth is typically red to reddish brown in color, spicy and bitter, and somewhat sweet, with about 32-36 proof.
Historically, sweet vermouth was also known as Italian vermouth, though many countries now make it. In 1786, Antonio Carpano, invented vermouth after noticing German wines flavored with wormwood to give them an herbal, tonic flavor. The Carpano company now makes Carpano Antica, an incredibly complex sweet vermouth with a strong dried fruit aspect. Carpano Antica is expensive and only sold in liter bottles.
Due to the presence of sugar, sweet vermouth is typically more sturdy than other styles. Still, you should always buy the smallest bottle you can, and drink the vermouth within a week or two.