Sites are logical reprenstation of the physical layout of the network which help orgonize active directory domain controller replications and organize the subnet. Every site must have one or more domain controllers and at least one bridgehead server automatically or manually assigned. For example, a business that have multiple physical locations connected by T1 line can replicate common Active Directory objects between the sites at specified time entervals.
Types of replciation
There are two types of replications, Intersite and Intrasite. Intersite replications occur between domain controllers located in sites that are physically seperated and connected through WAN link. Intrasite replication occur between domain controllers on the same site seperated by subnet.