Brandy is added to fermenting red wine. The high alcohol level of the brandy stops the fermentation and leaves behind unconverted sugar. The difference between various styles of port has to do with aging time and whether the wine is put into wooden barrels. Vintage port rarely sees wood but spends two years in stainless steel before bottling. Traditional wisdom dictates that it be allowed to reach maturity while cellared in the bottle.
Tawny port stays for at least six years in barrels before bottling.
The wood-aging period causes the wine to take on an amber or tawny color. The wine is ready to drink when it is bottled. Stopper corks go into all other types, including rubies and tawnies. People don't worry too much about cellaring vintage port for years. But after about five years, it begins to close up, the fruit mellows out and is not really terribly attractive.