What is the Internet?
The Internet is the global network of interconnected computer networks. There are millions of smaller networks, which are in turn connected and which together carry various information and services, such as email, chat, files, as well as Web pages and other documents of the World Wide Web.
Here's a definition of the Internet from Macquarie Dictionary.
How does the internet work? There's an overview at HowStuffWorks and a more detailed summary written by Ross Shuler.
There's also a detailed overview of the Internet on Wikipedia.