Coined by Robert Metcalfe, the inventor of Ethernet, Metcalfe's Law states that the value of a network grows by the square of the size of the network. The idea behind this law is that a network's value is increased as the size of the network increases; this law is often referred to when talking about the Internets value. For example, if the network has 5 machines its value would be 25 (5^2=25), but if another network had 1000 machines its value would be 1,000,000.
This law is also considered applicable to more than just the Internet or a computer network. For example, a software product may increase in value as it grows in size. If a product only has 25 users, it is less likely to be known and used. However, if the same product has 1,000 users, it is more likely to be known and used.