A reaction in which stereochemically different reactants give stereochemically different products is called stereospecific reaction. A reaction that yields predominantly one stereoisomer (or one pair of enantiomers) of several possible stereoisomers is called stereoselective reaction. All stereospecific reactions are necessarily stereoselective, but all the stereoselective reactions are not necessarily stereospecific
In this videos different stereospecific and stereoselective reactions have been discussed with appropriate examples.

Erythro and Threo nomenclature:
   • Erythro and Threo Nomenclature