What are identical molecules? I don't know what identical molecules are but these are by thoughts. 

Since from the word identical the molecules should have the same number and kind of atoms attached in the same way. Since in stereo-chemistry the isomerism is due to spacial orientation all stereo-chemical isomers are identical. 

Is my thinking correct? What factors determine whether two molecules are identical or not?
Identical compounds have the same chemical and physical properties (and other properties too). 

Of the two types of stereoisomers, diastereomers (non-mirror image stereoisomers) are clearly nonidentical. Consider the isomers of tartaric acid. Meso-tartaric acid and (+)-tartaric acid have different melting points (165 $^\circ$C and 171 $^\circ$C, respectively). They also have different acidities.

The (+) and (-) isomers (enantiomers) are more challenging, but they are distinguishable by one physical property - the way they rotate plane-polarized light. If the two molecules were identical, they would not be isomers - we would not be able to distinguish them. If they are distinguishable, they are not identical.