Definition
A server is a computer or program that provides services to other programs or devices (clients). The word covers both hardware — reliable, always-on computing equipment in data centers — and software, meaning a process that listens for and answers requests, such as a web server or database server.
How it works
In the software sense, a server keeps listening on a port, processes each client request and returns a response, commonly in a request-response pattern. In the hardware sense, servers are deployed in IDC facilities and serve clients over the network.
Usage
Loading a web page, sending an email or calling an API all come down to some server responding. In development, “starting a local server” means running a process on your own machine that listens for HTTP requests — nothing to do with expensive data center hardware.
Related and confusable terms
A VPS is a virtualized server rented to you; an app server is the kind of software on a server that hosts business logic; a network and the internet are the communication foundations that connect servers together.