Energy recovery linacs recapture the energy of the used electron beam into the electromagnetic field in RF cavities for subsequent acceleration, which can greatly reduce both the RF power supply required for high current acceleration and harmful radiation from the dumped beam. In addition to a high energy efficiency and reduced radiation from the dump, energy recovery linac light sources have the advantage of providing short-bunch, low-emittance electron beams for emitting high-brightness, highly coherent photons. These characteristics make them a very promising candidate for future advanced light source. This paper presents an introduction to energy recovery linacs, with the emphasis on basic principles and the most important physics and technical problems, as well as the activities in developing energy recovery linac facilities around the world. A brief introduction to some representative schemes of energy recovery linac light source is presented in the end.