Scale-free Network

In scale-free network, the very few network nodes (called "hubs") were far more connected than other nodes. The probability that a node connects with k other nodes was proportional to k^−γ, where γ is between 2 and 3. Many real networks fall in this group, such as Word Wide Web (Internet), protein-protein interaction, sexual partners in humans, social network and so on.
Barabási A.L. et al. wrote an review, named Scale-free networks, on Scientific American in 2003. Here is a general introduction on Wipipedia.