Linac-based X-ray free electron lasers (XFELs) are the fourth generation light sources consisting of an injector, a linac section including RF linearizer and magnetic bunch compressor and an undulator section for the production of FEL radiation caused by the interaction of an electron beam and an electromagnetic radiation field produced by the beam itself. The flexible use of XFEL radiation is based on variable focusing and tuning the X-ray energy. These methods allow one to change the power density of the incident X-rays by many orders of magnitude. Due to the unique futures of XFEL radiation like short intense pulses, fields of high amplitude, frequency, and coherence, it has wide range of applications in variety of research fields, Chemistry, Biology, Medicine, and ets,. XFEL principles, light properties and applications will be reviewed in the conference.