With increasing number of mobile phones and smart devices, it has become hard to manage the networks proactively as well as reactively. Software Defined Networking (SDN) is an emerging technology that promises to solve majority of the challenges faced by the networks in current times. SDN is based on decoupling of data plane and control plane. SDN has a generalized control plane for all networking devices of the network which makes it simple and easy to configure devices on the fly. This paper surveys how Software Defined Networks evolved to be one of the most preferred technology of contemporary times. The architecture and working of all the planes of SDN have been discussed. SDN finds application in variety of areas, some of which have been highlighted in this paper. SDN faces many security threats in each of its planes. The major security challenges are also presented in detail at the end of the paper.