What is Computers to which other computers connect called?

A Server

Depends on the network topology (design). Is the topology a token-ring, star, bus, etc? Server is usually the correct answer in general, but you also have switches, routers, and on older networks, hubs. Some old networks didn't use servers at all. All of the computers combined made up the "server" so to speak as they were all directly linked to one another such as in a bus topology. The problem with bus is that if on computer goes down, the entire network goes down.