True vintage Madeira, even 100-year old Malmsey Madeira, has a surprising amount of acidity. Genuine Madeira is produced on the island of Madeira; imitations made in other places lack the depth of flavor and balance of genuine Madeira.
There are several kinds of Madeira. Sercial is completely dry and, because of its acidity and lack of sugar, is little used in the kitchen. Verdelho is sweeter and softer than Sercial, which makes it useful for sauce making. Bual and Malmsey are the richest and sweetest types of Madeira. They can be used in sauces calling for Madeira—they are often the best—but use them carefully, because they can be aggressive; they are also expensive.