Servers are the computers in the network that perform some service or function for all workstations connected to the server. Server does not always need to be physical. Several virtual servers could be located on a single computer. Example of servers could be print server, storage server, or network server.
Typically server is a dedicated device which performs single task. Application defines the requirements for the server hardware. The networking hardware is the most important part of hardware, which defines access speed and bandwidth of the server.
Servers should have the 24/7 uptime. This point makes reliability and durability most important aspects of the server. It has to be built from parts with low failure rates. That is why server hardware is more expensive than the one for workstations. As a failure precaution servers are typically equipped with power battery that will allow it to operate during power failure. Bigger servers are having multiple independent power sources that would ensure necessary uptime. Some companies go further and install multiple cooling systems, hard drives, and power supplies as additional backup.
Main servers are connected to the core switches with high bandwidth connection. Modern networks would nearly always have optical fiber for that. Older networks might be using more conventional UTP copper cable. Main equipment room is typically the place where server racks are installed. This way better security and cooling could be provided.