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Enantiomers are a type of stereoisomer, which are molecules with the same chemical formula and connectivity but a different spatial arrangement of atoms. Specifically, enantiomers are pairs of molecules that are non-superimposable mirror images of each other. This means that if you imagine one enantiomer as an object and the other as its reflection in a mirror, you cannot rotate or flip one to perfectly align it with the other. They are like a left hand and a right hand; they have the same parts but are mirror images that cannot be superimposed.

The presence of a chiral center, typically a carbon atom bonded to four different groups, is usually what gives rise to enantiomers. If a molecule contains a chiral center, it will exist as a pair of enantiomers, provided there are no internal symmetry elements that make the molecule superimposable on its mirror image.

Enantiomers have identical physical properties such as melting point, boiling point, and density. However, they differ in their interaction with other chiral entities. This is most notably observed in their optical activity, where enantiomers rotate plane-polarized light in equal but opposite directions. One enantiomer will rotate the light clockwise (dextrorotatory, denoted by + or d), while the other will rotate it counterclockwise (levorotatory, denoted by - or l).

In biological systems, enantiomers can have vastly different effects. Enzymes and receptors are chiral, meaning they can distinguish between enantiomers. This leads to one enantiomer being biologically active and the other inactive, or even toxic. For example, thalidomide's tragic history illustrated this, with one enantiomer being a sedative and the other a teratogen. Therefore, in pharmaceuticals and other biological applications, controlling the stereochemistry and obtaining pure enantiomers is crucial.