Radiation is energy released in the form of waves and particles. Radioactive nuclei emit radiation as they return to stable states, either by ejecting a beta particle (an electron), an alpha particle (a helium nucleus), or a gamma ray (a high-energy photon). All of these forms of radiation can interact with other matter, and if the radiation is sufficiently energetic, this radiation can significantly change the states of their collision partner; such radiation is known as ionizing radiation. Humans are exposed to natural sources of ionizing radiation all the time on earth; this natural radiation is primarily the originates from the cosmos, aptly referred to cosmic radiation, and the presence of radioactive isotopes naturally present in the ground and in the atmosphere. Humans are also exposed to man-made sources of ionizing radiation, maybe most commonly through medical procedures and examinations. The scales of energy involved in radiation exposure, both natural and medical, span a surprisingly large range of energies.
Ionizing radiation is energetic enough to dislodge electronics from atoms and molecules, which can effectively kill cells, or spur mutations by altering the DNA of cells. The more radiation one absorbs, the more prone cells are to damage. Doses of ionizing radiation is measured in a variety of units; we'll consider the sievert (Sv) and gray (Gy). 1 gray corresponds to the absorption of 1 joule of energy per kilogram of matter; however, not all doses of gray are considered equal. 1 joule of radiation can do different amounts of damage to cells, depending on the type of cells effected and the type of radiation. A sievert is identical to a gray in its units, i.e., 1 Sievert is 1 joule of radiation absorbed per kilogram of matter, but its value is weighted by the type of radiation and the type of biological collision partner.  Furthermore, the damage inflicted by a radiation dose depends on the time over which it is endured, which makes comparing absolute values of dosages difficult in the units considered. Nonetheless, we can compare natural and medical doses of radiation in units of gray, as these leave the least amount of room for interpretation, in order to gain an appreciation for the scales of energy involved.
The average does of radiation absorbed by a human at sea level is about .27 mGy (miligray) per year, primarily attributed to cosmic radiation (which, in this case, has a conversion factor between Sv and Gy of about 1).  Because this cosmic radiation needs to penetrate the atmosphere, this value is a strong function of altitude. The average dose for someone living in Switzerland ranges between .34 and 1.66 mGy (the elevation of switzerland ranges from about 200m to 4,500 meters, with roughly 20% of the country lying above 2000m).  Furthermore, flight crews in the US are exposed to an additional 0.2 to 5 mGy per year depending on the number, duration, and altitude of flights flown per year.  Additionally, Radon exposure account for about 1 mSv of exposure, as reported by; radon is an alpha emitter and is inhaled as a gas into the lungs, and therefore its radiation is consider about 20 times more harmful than beta or gamma radiation.  Therefore, Radon exposure accounts for about .05 mGy of radiation dose per year using this conversion factor of 20.  The impact of these natural radiation doses on health is difficult to quantify, due to the number of other variables that may be relevant for each person, but at least these average doses give us a baseline for 'normal' levels of radiation exposure - about 0.35 mGy per year for someone living near sea level.
One can receive doses of radiation as part of medical examination or treatment. High-energy photons, i.e., x-rays, are used in plane film radiograms (colloquially known as "x-ray" images) and computed tomography (CT) scans. In either of these exams, a patient is subject to a burst of radiation. Only a region of interest is exposed to the radiation dose (e.g., a broken arm). Some of the radiation is absorbed by the body and the remainder of it passes through, exposing film, and producing an image of the region of interest, hopefully enabling the diagnosis of an ailment. Doses for these procedures range from about .15 to 15 mGy, depending on the patient what area of the body is being imaged.  CT scans employ the same principles, but require multiple x-ray images, which are reconstructed into "slices" of the region of interest. Again, depending on the region imaged, and the imaging scheme (e.g., slice thickness) between 20-70 mGy per scan. 
Radiation therapy for the treatment of cancerous tumors also utilizes a dose of radiation to kill the unwanted cells. A variety of forms of radiation treatment have been developed to reduce the damage done to neighboring healthy cells. Additionally, radiation fractioning, or splitting up doses into smaller doses over time, is also employed to enhance certain effects (e.g., palliation).  Doses for radiation therapies can be as high as 60 Gy, for which the radiated mass is equal to the mass of the irradiated cells.  For example, if a 250mg tumor was irradiated with a dose of 60 Gy, this could be considered equivalent to 150 mGy dose administered to a 100kg person, in terms of energy deposition. For comparison, a 1-2 Gy dose to the entire body results in acute radiation sickness and if not treated can lead to death; 8 Gy is typically lethal dose regardless of treatment. 
We see that certain medical treatments (e.g., plane film radiographs) are on par with natural exposure, but some treatments are up to 100,000 times larger than naturally occurring doses! Due to the complexities of radiation and its effects in and of themselves, and the huge number of variables that dictate the onset of caner (age, occupation, eating habits, etc.), it is difficult to precisely describe the consequences of natural or medical radiation doses. Yet, it's somewhat odd that some forms of cancer, which might be caused by radiation exposure, can be treated with much higher doses of radiation.
© Victor Miller. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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