X-rays are a type of highly penetrating radiation that damages fast-growing cells such as cancer cells. This is why x-rays are used in what is called radiation therapy. Unfortunately, x-rays can also damage normal cells, so an important goal of radiation therapy is to deliver as much radiation to the tumor as possible while keeping radiation to the rest of the patient's body at a minimum.
Most sources of x-rays produce "messy" beams whose rays point in different directions and spread out in an uncontrolled way. A collimator is like a "comb" for x-rays. It eliminates all the rays except the ones that are traveling more or less parallel in straight lines (see figure). A collimated beam of x-rays is much easier to control. With a collimated beam, the radiation therapist can deliver the most damage to the tumor while minimizing harm to normal tissue.
