Diastereoisomers:
Diastereoisomers are a type of stereoisomers that are optically active but not mirror images of each other. They have different physical and chemical properties and can be separated by traditional methods of resolution.

Optical Activity:
Optical activity refers to the ability of a molecule to rotate the plane of polarized light. Molecules that exhibit optical activity have chiral centers, which are carbon atoms bonded to four different groups. Chiral molecules exist as two non-superimposable mirror images called enantiomers.

Enantiomers:
Enantiomers are a type of stereoisomer that are mirror images of each other but cannot be superimposed. They have the same physical and chemical properties, except for the direction in which they rotate the plane of polarized light. Enantiomers are always optically active.

Mirror Images:
Mirror images are molecules that have the same connectivity of atoms but are arranged in a different spatial orientation. They are non-superimposable, just like our hands. Mirror images can be either enantiomers or diastereoisomers, depending on whether they are chiral or not.

Diastereoisomers vs Enantiomers:
The main difference between diastereoisomers and enantiomers is that diastereoisomers have different physical and chemical properties, while enantiomers have the same physical and chemical properties. Here are some key points to understand the difference:

1. Different Physical Properties: Diastereoisomers have different melting points, boiling points, solubilities, and other physical properties. This is because their three-dimensional arrangement differs, leading to different interactions with other molecules.

2. Different Chemical Properties: Diastereoisomers have different reactivities towards various chemical reactions. They may react differently with other molecules, resulting in different products.

3. Optically Active: Both diastereoisomers and enantiomers can be optically active. However, only enantiomers are always optically active, while diastereoisomers are not always optically active. In the case of diastereoisomers, the presence of multiple chiral centers leads to different spatial arrangements, resulting in optical activity.

4. Relationship with Chirality: Enantiomers are always chiral molecules, while diastereoisomers can be either chiral or achiral. Diastereoisomers that are chiral are optically active, while achiral diastereoisomers are not optically active.

Conclusion:
In summary, diastereoisomers are a type of stereoisomers that are optically active but not mirror images of each other. They have different physical and chemical properties, making them distinguishable and separable. Enantiomers, on the other hand, are mirror images of each other with the same physical and chemical properties, except for their optical activity.