Gamma Rays are very high energy EM waves. They are very dangerous as they can ionise molecules in cell DNA causing cells to mutate. This can lead to cancer. Because of this they are used to sterilise medical instruments by killing microbe cells.
They can also be used to treat cancer! The tumour is given a high dose to kill it but the rest of the patient must be given a low dose so that healthy cells aren't damaged. To prevent this, either the patient is rotated with the gamma source focused on the tumour or several safe doses are fired at the tumour that add together to make a dangerous dose.