Gamma rays are given off by stars, and by some radioactive substances. They are extremely high frequency waves, and carry a large amount of energy. They pass through most materials, and are quite difficult to stop - you need lead or concrete in order to block them out.

You can find out more about gamma rays at http://www.darvill.clara.net/nucrad/index.htm

Tracers:

Doctors can put slightly radioactive substances into a patient's body, 
then scan the patient to detect the gamma rays and build up a picture 
of what's going on inside the patient. 

This is very useful because they can see the body processes actually working,
rather than just looking at still pictures.  

Example of a tracer:

the picture on the right is a "Scintigram", and shows an asthmatic person's lungs. The patient was given a slightly radioactive gas to breathe, and the picture was taken using a gamma camera to detect the radiation. The colours show the air flow in the lungs.

In industry, radioactive "tracer" substances can be put into pipes and machinery, 
then we can detect where the substances go. 

This is basically the same use as in medicine.

Gamma rays kill microbes, and are used to sterilise food so that it will keep fresh 
for longer. This is known as "irradiated" food. 

Gamma rays are also used to sterilise medical equipment.

Gamma rays cause cell damage and can cause a variety of cancers.

They cause mutations in growing tissues, so unborn babies are especially vulnerable.