Dessert wines are sweet wines and as the name suggests, they are generally served with dessert. Since there are many dessert wine types, a rule of thumb for choosing a wine is that it must be sweeter than the dessert being eaten.
Fortified wine is a wine that has been blended with a liquor. The liquor most often used for this is Brandy which is essentially just distilled wine. This gives fortified wines a distinct flavor and a higher alcohol content than normal wine – usually at least %15 ABV.
A dessert or fortified wine glass should be smaller to direct the wine to the back of the mouth so the sweetness doesn't overwhelm. Dessert wines generally have a higher alcohol content, making small dessert wine glasses perfect for a smaller serving. The same rule of thumb applies to sherry and cordial glasses.