A server when discussing computers and networks is a system that shares information with other systems.
A server can be a computer or software. My home computer runs web server software. I have a server at work that runs software so that it has virtual servers on it. One of those servers can run a DNS server, mail server, and web server.
A computer that accessed a server is a client. Although servers can be clients of each other and clients can share info with each other.
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