Radiation Therapy, What is It?

So, Bug told us that she will be undergoing radiation therapy as treatment for her breast cancer. I've heard some stuff about radiation, but not much so I did some googling and found an awesome website for breast cancer information! This is what is says about Radiation Therapy:

Radiation therapy -- also called radiotherapy is a highly targeted, highly
effective way to destroy cancer cells in the breast that may stick around after
surgery. Radiation can reduce the risk of breast cancer recurrence by about 70%.
Despite what many people fear, radiation therapy is relatively easy to tolerate
and its side effects are limited to the targeted area

~http://www.breastcancer.org/treatment/radiation/