A pentose sugar is a sugar that has 5 carbons in it. Ribose and Deoxyribose are the two most commonly mentioned pentose sugars. (The only difference between Ribose and Deoxyribose is Deoxyribose, as its name suggests, has one fewer oxygen. If you took another Oxygen off it, it would be called DiDeoxyribose). Other sugars are classified by the number of carbons they have. Glucose, for example, with its 6 carbons is called a hexose sugar.