Aldopentoses

If we examine the general formula of an aldopentose, we see that they contain three asymmetric carbon atoms. 
This means that 2³ or 8 optical isomers are possible.  These are:
- D(–) xylose, L(+)-xylose, D(–) xylose,  L(–)xylose, D(–) arabinose, L(+)-arabinose,  D(–)-ribose, L(+)-ribose    

Aldohexoses
If we examine the general formula of aldohexose, we see that it contains four asymmetric carbon atoms. This means that 2⁴ or 16 optical isomers are possible. D and L forms of altrose, allose  glucose, mannose, galactose, talose, arabinose and idose
Only three of the sixteen possible aldohexoses are found in nature (all sixteen isomers have been prepared synthetically). They are D-glucose, D- mannose, and D-galactose. No one of these three optical iosmers is a mirror image of any of the others, so all three are diastereomers of each other.     

Epimers
A pair of diastereomers that differ only in the configuration about of a single carbon atom are said to be epimers. e.g D(+)- glucose is epimeric with D(+) -mannose and D(+) -galactose as shown below: