What is sites ? What are they used for ?

One or more well-connected (highly reliable and fast) TCP/IP subnets.

A site allows administrators to configure Active Directory access and replication topology to take advantage of the physical network.

A Site object in Active Directory represents a physical geographic location that hosts networks. Sites contain objects called Subnets.

Sites can be used to Assign Group Policy Objects, facilitate the discovery of resources, manage active directory replication, and manage network link traffic.