X-rays are a form of electromagnetic radiation, just like light, radio waves and gamma radiation, with a wavelength in the range of about 1 nm to 6 micrometer and a photon energy ranging from 1,2to 240 keV. They are characterized by their ability to pass through matter. Unlike light, X-rays are hardly affected by regular lenses etc. and propagate in straight lines.
X-rays are produced when accelerated electrons suddenly decelerate upon colliding with a solid body, since accelerated particles that are decelerated abruptly emit electromagnetic radiation.