The short answer is Radiation Therapist treats cancer patients and a Radiographer or Radiologic Technologist is an X-Ray tech.
These two jobs are related under the Radiology Banner but are VERY different. When you mention radiography/radiography technologist it is generally teh same thing. Many people start with that program and work and move into radiation therapy though unlike in the past it is now possible to go straight into radiation therapy. I personally would still recommend you start off as a tech.
More schooling is needed to get into Radiation Therapy.