These are the three possible sugars with three carbon atoms (trioses).
Glyceraldehyde is an aldehyde sugar because its carbonyl group is on the end of the carbon chain.
Dihydroxyacetone is a ketone sugar becuase its carbonyl group is on carbon number 2.
All sugars except for dihydroxyacetone have at least one chiral carbon and thus have stereoisomer.
Most natural sugars are sugars, which means their last chiral carbon has the same configuration as D-glyceraldehyde.
The mirror image (enantiomer) of a D-sugar is an L-sugar.