X-rays are very high frequency waves, and carry a lot of energy. They will pass through most substances, and this makes them useful in medicine and industry to see inside things.

X-rays are given off by stars, and strongly by some types of nebula.

An X-ray machine works by firing a beam of electrons at a "target". If we fire the electrons with enough energy, X-rays will be produced.

YouTube video clip: 1 min 15 seconds

YouTube video clip: 8 minutes 45 seconds
Rather long, but explains it really well.

X-rays are used by doctors to see inside people. 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

See also:
http://www.thenakedscientists.com/HTML/articles/article/bobburycolumn5.htm/

CT scanners use X-rays

See also:
http://www.thenakedscientists.com/HTML/articles/article/bobburycolumn6.htm/

X-Rays can cause cell damage and cancers. This is why Radiographers in hospitals stand behind a shield when they X-ray their patients. Although the dose is not enough to put the patient at risk, they take many images each day and could quickly build up a dangerous dose themselves.