X-rays have one of the smallest wavelengths in the electromagnetic spectrum. They have higher energy than ultraviolet light, but longer wavelengths than gamma rays. X-rays are primarily used for diagnostic radiography. They are formed through ionizing radiation, and too much exposure (the radiation is too intense and/or you are exposed for too long) can be dangerous.
Image acknowledgement: The University of Waikato Te Whare Wānanga o Waikato