Kate's cancer announcement: What do we know about her health‪?‬ What's That Rash?

Kate Middleton announced via a video message that she is undergoing treatment for cancer. 

The Princess of Wales has been out of the public eye since Christmas. This is the first official announcement about her condition since she revealed she would be undergoing planned abdominal surgery in January. 

So what What do we know about Kate's cancer diagnosis? What is the likely treatment? And, given she is relatively young, what's the message for people who do have unexpected new symptoms?

What is preventative chemotherapy? What could treatment look like? What are some of the side effects of chemotherapy? How long does it take to recover from? We're answering the health questions everyone's asking on What's That Rash?