This Web Basics class covers how the web works, as well as an overview of the elements of web browsers and web pages. The attached PDF document includes slides from the class.
This quick explanation of how the web works will only take five minutes of your time, and leave you with a wealth of information.
This site has been called "the cutest way to explain how the internet works!" (by librarian Heather Terrell). Check out the comic strip for a simplified explanation of what happens when you type a website address into your browser.
The Opte Project has been trying to map the internet since 2003. This map provides you with an idea of the vast scope of the web.
Entering the World-Wide Web: A Guide to Cyberspace was written in 1993, when the web was just coming into wider usage by the public. Although it's a couple of decades old, it still provides a good basis in what the web is. It's a more in-depth overview of the concepts underlying the internet and the web than the other resources recommended here.
This is an exhaustive list of keyboard shortcuts that can be used on the web to make browsing much more efficient, no matter which browser you're using.