A server is a computer designed to process requests and deliver data to other (client) computers over a local network or the internet. Although any computer running special software can function as a server, the most typical use of the word references the very large, high-powered machines that function as the pumps pushing and pulling data across the internet.
Most computer networks support one or more servers that handle specialized tasks.
As a rule, the larger the network—in terms of clients that connect to it or the amount of data that it moves—the more likely it is that several servers play a role, each dedicated to a specific purpose.
Strictly speaking, the server is the software that handles a task. However, the powerful hardware that supports this software is also usually called a server because server software coordinating a network of hundreds or thousands of clients requires hardware much more robust than what you'd buy for ordinary consumer use.