Depending on the kind of grape used, four varieties are available:

Sercial is the driest Madeira. It is light and has a slight almond aroma, and should be left to mature for at least ten years.

Verdelho is medium dry and offer nut and honey aromas with smoky, bittersweat tones. Its fine, piuant acidity and maintained with age.

Boal or Bual is a golden yellow, full-bodied, medium sweet Madeira, which can be stored for many years. Unlike the other varieties, though, it can also be drunk when young. It boasts a currant and dried fruit aroma and refreshing acidity.

Malvasia or Malmsey is the sweetest Madeira. Dark brown, very flowery, full-bodied and bold, it offers aromas of vanilla, caramel, coffee, honey, sweet almonds and candied fruits.

To produce Madeira, the fermentation of the grapy must is halted by the addition of a high-strength brandy. This gives the wines their characteristic sweetness, and it is only in the production of dry Madeiras that the alcohol is added after fermentation.

The wine is then warmed in temperature-controlled steel tanks at 45 to 50°C for three months. Better varieties are left for up to twelve months in 600-litre oak casks placed in heated rooms, while the highest-quality Madeirs are left for years or even decades in casks below roofs. Here they are warmed by the sun in the daytime and left in the cool in the evening.

This warming of the wine accelerates the maturation process and partly caramelises the sugar, giving Madeira its typical colour, smoothness and balance. 

Regardless of whether it is dry, medium dry, medium sweet or sweet, the wine is classified according to its age:

Fine = 3 years old
Reserve = 5 years old
Old Reserve oder Special Reserve = 10 years old
Extra Reserve = over 15 years old
Vintage oder Frasqueira = 20 years old

Sweet Madeira should be served at a temperature of 16 o 18°C, and medium sweet at 13 to
16°C. Dry Madeira are best enjoyed lightly chilled at 9 to 12°C.

A good madeira can be stored for 100 years if the cork is replaced every 15 years or so, while an open bottle will keep for up to 18 months without any loss of quality.

But why would you possibly want to wait as long as that?