X-rays are used by doctors to see inside
people. The machines are managed by a trained x-ray
technician. They pass easily through soft tissues, but not so easily through bones.
We send a beam of X-Rays through the patient and onto a piece of film, which goes dark where X-Rays hit it.
This leaves white patches on the film where the bones were in the way.
Sometimes a doctor will give a patient a "Barium Meal", which is a drink of Barium Sulphate.
This will absorb X-rays, and so the patient's intestines will show up clearly on a X-Ray image.
X-Rays are also used in airport security checks, to see inside your luggage. They are also used by astronomers - many objects in the universe emit X-rays, which we can detect using suitable radio telescopes.
Lower energy X-Rays don't pass through tissues as easily, and can
be used to scan soft areas such as the brain.
http://www.thenakedscientists.com/HTML/articles/article/bobburycolumn5.htm/ for how x-ray pictures work, and http://www.thenakedscientists.com/HTML/articles/article/bobburycolumn6.htm/ for CT scans and nuclear medicine.