They are extremely high frequency waves, and carry a large amount of energy. Gamma Waves can also pass through most materials, and are quite difficult to stop. Only some materials like lead or concrete can stop the waves.
As gamma waves can kill living cells, it is used in cancer treatment so that surgery does not need to be used. This is called "Radiotherapy", and works because cancer cells can't repair themselves like healthy cells can when damaged by gamma rays.
Another use is Targeted Radiotherapy, where a radioactive substance is used to kill cancer cells - but it's a substance that'll be taken up by a specific part of the body, so the rest of the body only gets a low dose. An example would be using radioactive iodine to treat cancer in the thyroid gland.
Doctors can also 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 allows doctors to see the body processes actually working, rather than just looking at still pictures.
This Scintigram 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.