For a very long time, I had a somewhat clouded idea of what servers were... some kind of special distant computer? Turns out it was in the name all along and a server is a machine that serves you with things. It could serve you a website, data you can download or other stuff. It can be near or far away.
It is this relationship that decides a machine is a server. In order for the machines to communicate they need a protocol, like a menu that gives them a shared language. A common protocol that we all use when surfing the web is the Hypertext Transfer Protocol or HTTP.
We have evolved hardware to be very good at serving and it has made the distinction between different types of machines larger. The image of a data center with servers just doesn't look like a personal computer.
But really, any computer can be a server. The thing that makes a machine a server is: does it serve? You can make your laptop into a server, you can make your friend’s computer into a server. You can even put a virtual machine within your own computer and turn that one into a server. Quite a mind-bender.
Of course, things always more complex when you dive in further but I find this helpful when I think about servers.