Madeira is a protected designation of origin (PDO) for liqueur wines from the island of Madeira, which is part of Portugal. It is a sprinkled wine, i.e. enriched with brandy. The alcohol content is between 17% and 22% by volume, depending on the variety. Madeira is a partially oxidized wine.
The different grape varieties will always give you an idea of how dry or sweet the Madeira is.
Sercial => dry
Verdelho => halfdry
Bual => halfsweet
Malvasia => sweet