Aldehyde sugars have an aldehyde group (-CHO) on the first carbon atom, while ketone sugars have a keto group (=O) on a carbon other than the first carbon.

This difference is reflected in the way that these two types of sugars are named. Aldehyde sugars are named by adding the suffix "-ose" to the name of the aldehyde, while ketone sugars are named by adding the suffix "-ulose" to the name of the ketone.

For example, the simplest aldehyde sugar is glyceraldehyde, which has one aldehyde group on the first carbon atom. The simplest ketone sugar is dihydroxyacetone, which has a keto group on the second carbon atom.

Aldehyde sugars and ketone sugars are both important in biology. Aldehyde sugars are used as a source of energy, while ketone sugars are used in a variety of other processes, such as cell signalling and protein glycosylation.