A server is just a computer like any home computer but often a little more powerful. The job of a server is to process and transfer data and on the internet they act like switchboards, transferring data from one computer to another.
A web server is the same but the content to be transferred is already on that server. You ask it for a web page and it transfers it to your computer. A web server will have an address called an IP (Internet Protocol) address. Every computer on the internet has an IP address.
Information is sent in packets, each with the destination’s IP address on them rather like postcards with a postal address on them.
There are other kinds of servers such as mail servers for all the email buzzing around the web. They all perform a service that the machine is normally dedicated to.