server can refer either to hardware or software.

A server is a computer used store large amount of data and deliver it as fast as possible to multiple clients. To complete this task, the computer uses server software.

The server software provides client access to network services. It provides data, services and information to another server.

When a lot of users visit a website at the same time and the request information from the server, the server can break down if it's not powerful enough. In this scenario, the requested information won't be displayed anymore in the client.