SI RADIATION MEASUREMENT UNITS: CONVERSION FACTORS
What's the Difference Between Roentgen, Rad and Rem Radiation Measurements?
A: Since nuclear radiation affects people, we must be able to measure its presence. We also need to relate the amount of radiation received by the body to its physiological effects. Two terms used to relate the amount of radiation received by the body are exposure and dose. When you are exposed to radiation, your body absorbs a dose of radiation.
Source: stevequayle.com