Diastereomers are a type of stereoisomer that are not mirror images of each other. They occur when molecules have multiple chiral centers, leading to different spatial arrangements of atoms. Unlike enantiomers, which are always in pairs, diastereomers can exist in groups of three or more. These compounds often have different physical and chemical properties, such as boiling points, melting points, and reactivity. This distinction makes diastereomers important in fields like pharmaceuticals, where different diastereomers of a drug can have varying effects on the body.