Stereoisomers are a type of isomer that have the same molecular formula and connectivity of atoms, but differ in the spatial arrangement of their atoms or groups. This means that stereoisomers have identical chemical properties, but different physical properties and biological activities.
There are two types of stereoisomers: enantiomers and diastereomers. Enantiomers are mirror images of each other and have the same physical and chemical properties, but they rotate plane-polarized light in opposite directions. Diastereomers, on the other hand, are stereoisomers that are not mirror images of each other and have different physical and chemical properties.
The existence of stereoisomers is due to the presence of chiral centers in a molecule. A chiral center is an atom in a molecule that is bonded to four different groups, which results in two possible spatial arrangements of the atoms around the chiral center. If a molecule has more than one chiral center, it can have multiple stereoisomers.
Stereoisomers are important in many areas of chemistry, including drug design, biochemistry, and materials science. In drug design, for example, the different biological activities of enantiomers can lead to different therapeutic effects, while in materials science, the different physical properties of stereoisomers can be used to create new materials with unique properties. Stereoisomerism is also an important concept in organic chemistry, and understanding it is crucial for predicting and explaining the reactivity and behavior of molecules in various chemical reactions.
are isomeric compounds. 
are functional group isomers.