Isomers can be divided into 2 main category, which are structural/constitutional isomer and stereoisomers.

Structural (constitutional) isomers are isomers with same molecular formula but with atoms bonded in different order .
3 structural isomers of pentane , C5H12.

The atoms of stereoisomers are joined in the same order but have a different spatial arrangement . Geometric isomers are one form of stereoisomers due to restricted bond rotation.

are chiral molecules that are non-superimposable mirror image.
This is an example of superimposable mirror image.

Our hands are non-superimposable mirror image.

In general, a molecule with non-superimposable mirror image have 4 different substituents at the chiral center. These 2 mirror image are called enantiomers.