Everything is identical about these twins. Number of atoms, type of atoms are same. Only difference is the spatial arrangement of atoms or groups of atoms attached to the carbon atoms. There is one more trait of these twins-they are mirror images of one another. They are called stereoisomers. Stereo means spatial.
There is a slight differences in the reactivity of these members. One very interesting property of these molecules is that they rotate the light passing through their solution in equal and opposite directions called Levo and Dextro.
When such compounds are synthesized in the laboratory, we obtain both isomers in equal amounts and there is no optical activity because effect of one type isomers is nullified by the effect in opposite direction.
But in nature, synthesis of macro-molecules like carbohydrates, lipids and oils by the biological enzyme driven reactions we exclusively obtain only one kind of isomers-mostly levorotatory. This is due to enzymes which are proteins and very specific catalysts whereas industrial catalysts are surface active and both isomers are manufactured in equal amounts. Special techniques are required to separate them into pure Levo or Dextro isomers. Due to this reason the cost of these isomers increases manifolds. It has been found that some diseases respond to specific isomers. Such medicine are called stereo-specific medicine.