The D isomer is one of two forms of optical isomer, arising from the presence of a chiral carbon, which can rotate plane polarized light either clockwise or anticlockwise. 'D' stands for dextrorotary, meaning that the isomer rotates plane polarized light clockwise.
In molecules where more than one Chiral centre is present, the assignment of D or L is given according to the arrangement of functional groups on the highest numbered chiral carbon.
A well-known example of the difference between D and L isomers is that of Thalidomide, which was in the past given to pregnant women to counter the effects of morning sickness. The drug was given as a racemic mixture, which contains equal amounts of L and D enantiomers, therefore counteracting the ability to rotate plane polarize light in either direction, and while the D isomer was effective at curing morning sickness, the L isomer caused severe limb abnormalities in the developing foetus. Thousands of children were born with abnormal or underdeveloped limbs, and thalidomide was banned in many countries. Since the thalidomide disaster, all chiral molecules are rigorously tested before being marketed to the public, and racemic mixtures are very rarely sold.