Some of you might say: well, you have just answered your own question, X-ray is a form of radiation. However, nothing is always as simple as it seems. Any type of elementary particle emission is called radiation, while X-ray is a type of electromagnetic radiation, meaning photon emission, namely light.
Optical light to which a typical human eye normally responds corresponds to the electromagnetic wavelengths located between 380 to 750 nanometers, with a maximum sensitivity at 555 nanometers. Against general belief, this specific wavelength is attributed to the green light, not to the red one! Thus, the human eye is more sensitive to the green color than to red. X-ray, on the other hand, is located in the electromagnetic spectrum between the wavelengths 0.01 to 10 nanometers, and, just like infrared, microwave and radio waves, this type of light is invisible to the human eye.
X-ray light is also more energetic than the previous types of electromagnetic radiation enumerated in the upper paragraph, but not the most energetic in the electromagnetic spectrum. Gamma ray has the highest frequency, energy and the shortest wavelengths inside the electromagnetic spectrum and it can pack such high energy that can actually create extensive damage in living cells.
Nonetheless, a single X-ray photon can have hundreds to thousands of times the energy of a photon of light in the optical spectrum. They were first discovered by German physicist Wilhelm Conrad Roentgen in 1895, who later received the Nobel Prize for physics for his finding that quickly got to be used for medical purposes.
X-ray light is being routinely emitted throughout the universe as highly energetic atoms experience a decay that returns them to the ground state energy configuration, when time electrons are being ejected. In the same time, the atom will also emit a X-ray photon in order to shed enough energy to return to the ground state. The process is commonly known as 'braking radiation'. In X-ray generating devices on Earth we are capable of producing X-ray radiation by smashing electrically charged particles, such as electrons, into metal targets.
Usually, the metal targets are constructed out of tungsten, which emits X-ray radiation through a 'braking radiation' process, as an energetic beam of electrons is decelerated into the tungsten target. Not only that, but electron-tungsten atom collision produces X-ray radiation through two separate mechanisms, one by electron-atom collision and the second by electron emission from the tungsten atoms.
Alternatively, particle accelerators, such as synchrotron, can produce powerful beams of X-ray light by accelerating electrons at relativistic speeds. As they are being spun throughout the whole circumference of the synchrotron, electrons receive energy continuously, so that they reach higher and higher speeds. However, electrons cannot surpass 99.9 percent of the speed of light and they would eventually have to shed the excess energy through X-ray light emission.