If you have folders on your computer that are 'shared' with other computers, your computer is acting like a 'server' for those files.
A 'server' is a very, very general designation. You can put a 'web server' on your PC using, say, nginx or apache2. You won't get traffic unless you have a public IP address, but you can display pages for your home network.
There are 'file servers', 'ssh servers', 'mysql servers', etc, etc, etc. Any time your computer offers a service to another computer, it is a 'server'.
A 'server' is a very, very general designation. You can put a 'web server' on your PC using, say, nginx or apache2. You won't get traffic unless you have a public IP address, but you can display pages for your home network.
There are 'file servers', 'ssh servers', 'mysql servers', etc, etc, etc. Any time your computer offers a service to another computer, it is a 'server'.