Because of their small wavelength, they are able to penetrate almost everything, flowing through electrons, but not transparently through hard matter. This is why these are commonly used in radiology (by doctor or dentist) Though penetrating matter, on a smaller scale, cells and DNA can absorb them and thus be damaged. This is why they are used in radiotherapy, where a certain doze is used to kill cancer cells for example.
X-rays are primarily used for diagnostic medical imaging and crystallography
Gamma rays are now usually distinguished by their origin: X-rays are emitted by definition by electrons outside the nucleus, while gamma rays are emitted by the nucleus
The X-ray band starts where all atoms and molecules can be ionized.
Hot gases in space, Sun, X-Ray tubes
röntgen scanners (medical, archeological, ..)
physicist's detectors for analysis of matter
Interaction with matter:
ionising: the seperation of electrons from it's atomic bounds which can be unhealthy in high doses (hence the protections shields for radiologists)