Chiral comes form the Greek word cheir meaning hand. Just as your hands are nonsuperimposable mirror images, so are chiral molecules.
Chiral molecules are isomers.
Isomers are compounds with the same formula, but contain a different arrangements of atoms and different chemical and physical properties. Structural isomers are compounds that contain the same atoms but the bonds within the compound differ. Stereoisomers have different spatial arrangements of the atoms even though they contain the same bonds. One type of stereoisomerism is optical isomerism. Optical isomers have opposite effects on plane-polarized light.