In a nutshell: Exceptionally long-lived, tangy, fortified wines.
Main grapes: A sorry tale.
Madeira is a volcanic island in the middle of the Atlantic, a wine, and a miracle. Good-quality madeira (which, alas, can cost at least as much as classed-growth bordeaux) is the world's longest-living wine. Its high alcohol and natural acidity and the fact that it has been virtually boiled during the production process mean that it is indestructible, whether in a wooden cask or glass carboy maturing under the rafters of a lodge in Funchal or in a bottle at home, however long ago it was opened. Madeira is the only wine that can be kept in a decanter on a sideboard for months without deteriorating. I also reckon that this unique fortified wine also has the strange property of protecting the drinker, well this drinker anyway, from a hangover (unlike port).
The trouble is that most of the madeira produced today is rather ordinary - very good for cooking and adding richness to meat reductions, but hardly fine enough to savour by the thimbleful. This is partly because so many of the noble grape varieties Sercial, Verdelho, Bual and Malvasia (for Malmsey) were replaced by the easier-to-grow, much more prolific but coarser Tinta Negra Mole. A shortcut to seriously good-quality madeira is to look for one of the four noble varieties on the label. Wines made from Tinta Negra Mole are labelled by age (5 year old, 10 year old or 15 year old) and style (see below).
Sweeter madeira is made very much like port in that it is fortified halfway through fermentation while drier styles are, like sherry, fermented fully to dryness before grape spirit is added. But madeira's real distinction is that the young wine is subjected to extreme heat, either artificially in a heated vessel or more naturally and slowly in hot lofts, which is where its deep caramel colour comes from.
Understanding madeira labels
Bual/Boal, rich, dark, sweet madeira that ages well but can be drunk younger than Verdelho and, especially, Sercial. Drink with or after cheese. Lesser imitations made from Tinta Negra Mole are labelled Medium Sweet.
Malmsey, the sweetest, darkest madeira, for drinking after a meal. Lesser versions are labelled Sweet or Rich.
Sercial, the driest style of madeira which can make an extremely fine aperitif but demands many decades of ageing before it reaches its full potential. Lesser versions are labelled Dry.
Solera, rare madeira that is the produce of gradual, fractional blending like sherry in an ageing system that began in the year specified. (A madeira labelled Solera 1870 comes from an ageing system established in 1870, for example.)
Verdelho, the second-lightest, driest madeira with a delicious tangy nuttiness. Lesser versions are labelled Medium Dry.
Vintage madeira, a (very rare) madeira labelled, say, 1944 without any mention of the word Solera should contain nothing but wine harvested in 1944.