What are millions of computers around the world connected by a network of networks?

The internet, or the World Wide Web as it's called also, originally was used by academic researchers at universities across the country to more easily share data on studies or common areas of interest. DARPA (now ARPA, the US Military's Advanced Research Project Agency) liked the idea of a web-like network that was self-healing if part of it was compromised. Data was re-routed if it couldn't make it from its point of origin to its destination in the most efficient manner.

So the answer is to the question is a WAN, a Wide Area Network (as opposed to a household router with three computers relying on it, a (Local)AN. The World Wide Web rides on the Internet, but generally a mechanism takes human-friendly domain names and converts them to a series of digits that for some reason computers like more when sending and receiving packets of data.