Server
In common parlance, the word "server" has two meanings.
Software (or hardware)
A server is software or device that provides a service to another software or device, also known as the client.
In the context of the Fediverse, a server is a running instance, that will hold one or more Fediverse accounts.
Example
- the software that you use to read emails is the client, and the server is where you will be retrieving all received emails from
- A Mastodon instance is a server; provides to you access to the website
Computer
A server is a physical or virtual computer that is used to host software.
The computer is almost similar to a desktop or laptop computer, but can be optimized for running Internet software.