If you try to put the two molecules on top of each other, they will not be the same (they will not superimpose). This is because they have one chiral centre. It is important that they only have one, otherwise the two molecules are called diastereoisomers.
Both enantiomers of a molecule have the same basic properties. For example, they have the same boiling point and NMR spectrum. Therefore they are difficult to separate. It is sometimes very important to separate them because in some drug molecules one enantiomer might have the right effect but the other might be dangerous. One famous example is thalidomide: one enantiomer of the molecule causes birth defects.
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