The correct matches are as follows:
- Site: A subnet or subnets connected by a high-speed link
- Site link: A mapping in Active Directory of the connection between sites
- Subnet: A physical segment of the network that uses IP addresses derived from a single network ID
- Domain: The basic security boundary in a Windows network, which has its own security policies
- Forest: A group of domain trees that share a noncontinguous namespace
Site links allow you to define what sites are connected to each other and the relative cost of the connection.
When you create a site link, you specify which sites are connected by this link and what the cost or metric of the connection is in a relative-costing model. For instance, three sites A, B, and C could be connected to each other, but because you understand the underlying physical network, you might feel all traffic should be routed through the A site.
This would require you to configure to two site links: A to B and A to C.
If at a later time additional physical network connections were established between B and C, you could set up one more site link and connect B and C together. If you configure all three site links with an equal cost say, 100 traffic could then flow across the B-to-C link instead of from B to A to C.
This is because the total cost to use the B-to-C link would be 100 and the total cost to use the B-to-A-to-C route would be 200, which is more expensive.