I was watching something on the Chernobyl nuclear disaster and noted they were using Sieverts to measure exposure. But I have seen more units used to measure radiation. I am not sure this is a complete list but I can think of (in no particular order):
All that seems to confuse the average Joe. I am sure there is a reason for all those (and others I may be missing). What is their difference?
The Gray and the Rad are the absorbed dose in joules /kg for the Gray and ergs per g for the Rad, so the same thing but different units. It is a measure of how much radiation energy has been absorbed into something. The roentgen is more or less the same thing just different units and a measure of how much air is ionized which relates to energy absorbed per kg.
The Sievert and the Rem are a measure of how much harm that will do or the equivalent dose, based on the type of ionizing radiation that deposited the energy. There is a quality factor or weighting factor for gamma rays it is 1, alpha particles are 4 and fast neutrons are 20 if I remember correctly . A joule per kg of energy into your body from neutrons is 20 times more likely to cause bad things to happen than a joule per kg of gamma rays.
The Seivert or Rem is a probability ( or stochastic) measurement of the long term health effects from ionizing radiation. What the radiation measuring device you are using and what it shows on the scale are really a matter of how it was calibrated rather than measuring something different at the sensor level.
When you get to things like non stochastic effects such as radiation burns the measure of sieverts in the area you are in is going to tell you how bad things are but may not scale with the lower dose rates and probability of long term health effects .
When we talk effective dose there are further subtleties according to if it is an external source or ingested inside your body and there are terms like effective dose and commuted dose but no need to dive into that. Life time and short term effective does also has to be considered.
There are many different types of radiation detectors, your classic Geiger tube, germanium crystals, film badges ( which can have shielded parts to differentiate radiation type sort of), neutron detectors which can be the classic Geiger tube in a big polyethylene sphere or helium tubes . What they show on the scale Seivert Gray , REM etc is largely a matter of how it is calibrated to the physical measurement.
Edited to clear up typos, fat fingers and equivalents not effective dose.
While it is occasionally useful to know what will happen to humans who get too close to unshielded nuclear piles, let’s not forget about your basic unit of radioactivity: 1 decay per second (= 1 Bq) = 2.703 × 10−11 Curie (who also got the Nobel Prize; her unit is based on the activity of 1 gram of radium)
Not too surprising in dosimetry, as this is an SI unit
Should Curie be added to the list?
Becquerel?
The instrumentation and dose rate measurement is designed for low does rates ( that’s a whole other matter of perspective as to what you consider low) to measure and mange long term health effects for people working around radiation . When the dose rates are multiple sieverts per hour it really doesn’t matter too much about the long term health effects, but obviously you would want to know they dose rates are very high so you know to get out of the area, fast . The golden rules of radiation protection are Time, Distance, and Shielding.
So measurements of absorbed and effective dose are primarily what you need , the activity in Bq or Ci is not really that helpful , if radiation protection is what you are concerned about .
The activity of a radioactive object , the number of disintegration per second (bq) is a factor , but energy absorbed is what hurts you . So you need the detector to not only count the number of ionizing particles it picks up, but you also need to know what energy they have by the time they get to where the detector is, and what type they are to asses the long term physical risk. Hence you measure Gray not Bq.
“rad” is the CGS unit for absorbed dose; “gray” is the SI unit. The SI units are preferred, but “rad” is used in many places. It is common to see “cGy” used instead of “Gy”, which is handy because 1 cGy = 1 rad.
A similar relationship exists between “rem” and “sievert” where 1 Sv = 100 rem.
The difference between the raw physical units describing radiation emitted by a radioactive material (measured in units like curies and becquerels), versus measurements designed to reflect the different amounts of radiation energy absorbed by a mass of material (measured in rad or gray), and those that measure the relative biological damage in the human body (using rem and sieverts), which depends on the type of radiation.
Wonderful explanations =)
Now would anybody like to see a 56 grey radiation burn? 
That highlights where you may uses Sieverts ( effective dose) or Gray ( absorbed dose).
The sievert is generally used for lower dose levels and used to asses long term health implications ( stochastic effects) and life time dose is measured in Sv.
When you receive hefty doses in, hopefully, medical procedures the Gray is used as tissue damage ( good old non stochastic effects ) becomes a more important issue to measure and asses and the j/kg is a better measure . There will be calculations on whole body does, organ dose etc. 56gray to a targeted area isn’t the same as 56 Sv effective dose . The former will burn stuff up but is 56J/kg on maybe 0.1 kg of organs. the latter, assuming it is gamma rays would be 56j/kg for each kg of your body. That would be a quite unhealthy amount.
ETA Pics or it didn’t happen !
Well maybe not
It’s been a long while since I worked in Health Physics but in the plant ( it had a bunch of experimental reactors on the site) other than various detectors for spotting contamination, we had dose rate meters for general exposure in micro or milli Sv /hr. I don’t remember if the big emergency ones were, other than a higher range on the scale and detector physics that wouldn’t saturate, in Sv or if they switched to Gray , either post fact calculation of effective or absorbed dose would have been relatively straight forward from what ever was measured with the context of the errors .
I did some reading and YT viewing recently about nuke plants and accidental releases such as at Fukushima and Chernobyl.
The total amount of material released outside of the plant in each case could be measured in large numbers of Bq, peta- or tera-, as in, how much released into water, the atmosphere, etc.
Then, the total release of each could be compared, e.g., Fukushima’s total release was 10 to 15% that of Chernobyl.
If all all y’all really want plenty of pics, feel free to peruse the stomach-churning PDFs on this page: Publications Advanced Search | IAEA