A web server takes a client request and gives something back to the client.
A web browser lets a user request a resource. The web server gets the request, finds the resource, and returns something to the user. Sometimes that resource is an HTML page. Sometimes it’s a picture. Or a sound file. Or even a PDF document. Doesn’t matter—the client asks for the thing (resource) and the server sends it back.
Unless the thing isn’t there. Or at least it’s not where the server is expecting it to be. You’re of course quite familiar with the “404 Not Found” error—the response you get when the server can’t find what it thinks you asked for.
When we say “server”, we mean either the physical machine (hardware) or the web server application (software). Throughout the book, if the difference between server hardware and software matters, we’ll explicitly say which one (hardware or software) we’re talking about.