What is the difference between a server and a client in a network?

A server provides information/services; a client requests them — and the same device can play either role depending on the exchange.

Server and client are roles in a conversation, not fixed kinds of hardware. A computer is a server whenever it runs software that answers requests, and a client whenever it runs software that makes them. That's why the same machine can be both at once — your laptop is a client when it browses the web, but a server when it shares a printer. What decides the role is the software running, not the box.

* Client and server are roles in an exchange — the client requests, the server answers — not fixed kinds of hardware. *

• Server: A computer that provides information or services to end devices (e.g., email servers, web servers, file servers)
• Client: A computer that sends requests to servers to retrieve information

Go deeper:

• Client–server model (Wikipedia) — spells out the request–response pattern (clients initiate, servers await/answer) with concrete examples.
• Key Players (Practical Networking) — goes deeper on hosts/clients/servers as roles rather than fixed hardware, and how they interact.