A Network of Networks
The simplest way of explaining the Internet is to call it "the network of networks." It's the connection of computer networks around the world into one entity, so to speak. It's not one big computer, but rather numerous networked computers connected together.
When you dial into your Internet service provider (AOL, Earthlink, etc) from home, you are essentially connecting your computer to a network. If you are on campus you connect to the Internet through your school's network, which is connected to the larger Internet network through Peachnet, which is the electronic highway for all educational institutions and libraries throughout the state of Georgia. The "backbone" of all these connections is what you might hear referred to as the "information superhighway."
Map Source: (link will open to a pop-up window) Visual of network connections in North America