X-rays are a type of radiation. They are very similar to gamma rays, and differ only in the way they are produced.
Radiation is a general term that refers to any sort of energy that can travel through space as either a wave or a particle. Examples of other types of radiation include visible light, radio waves, and microwaves.
X-rays are similar to light, except that they have a much higher frequency, meaning they can pass through the human body. This makes them both invisible to the naked eye and ideal for looking inside the body. It also gives them the ability to damage cells through a process called ionisation.
Medical Physics is involved with ensuring that the doses to people from X-ray examinations are kept as small as possible, thereby minimising any chance of harmful effects. In fact many medical X-ray procedures involve doses similar to those you would encounter during everyday life.
A photograph illustrating the use of a test object to assess image quality in a general X-ray room.