Radiation is related to contamination in the same way that odor is related to manure and fertilizer. The following analogy illustrates this point. If you stand next to a freshly fertilized field, you can smell the fertilizer (manure). When you walk away from the field, you leave the odor behind. If you had stepped into the field, you would carry some of the fertilizer (manure) away with you on your shoes, and may be able to smell the odor of fertilizer until you clean your shoes.

In the same way, if you stand next to radioactive material, you will be exposed to radiation. As long as you don't get the material on your body or clothes, you can walk away and leave the source of radiation behind. If you get the material on your body or in your body, you become contaminated. You will continue to be exposed to radiation until you wash the material off of your body. The radioactive material continues to emit radiation, but you are no longer carrying it with you.