In recent years many properties of real world networks have been studied. Examples include the Internet, the WWW, social networks and cellular networks. It has been shown that these networks are not completely random, and do not obey the classical Erdos-Renyi random graph theory. Rather, they have a scale-free characteristic, i.e, the number of links per node has a power-law distribution. Here we present some of the properties of scale free networks. We discuss the robustness of scale-free networks to random breakdown of nodes, and their fragility to intentional attacks. We present the ultra small world properties of scale free networks, and describe some applications such as a method for efficient immunization of populations and computer networks.