From Wikipedia
A stack of servers
A server is also the person who serves you, like the waiter at the restaurant.

In net jargon, the group of words people use about the Internet, the server is software that runs on the cig computer to serve many things from user or client machines. Client-server is the usual name for this. It relies on the protocol. For example an HTTP server runs the HTTP protocol to create what you see as the web site on your computer.

In the peer to peer system, every computer might be both the client and the server to each other. This is good for file sharing.