The sievert (symbol Sv) is an SI derived unit of equivalent dose or effective dose (of radiation), and so is dependent upon the biological effects of radiation as opposed to the physical aspects, characterised by the absorbed dose (measured in Grays).
The equivalent dose to a tissue is found by multiplying the absorbed dose by a "quality factor", dependent upon radiation type. The effective dose to an individual can then be found by multiplying the effective dose by a factor dependent upon the part of the body irradiated. It is in some respects unfortunate that these two somewhat different quantities share the same unit.
1 Sv is equal to 100 rems.
The natural background effective dose rate varies considerably from place to place, but typically is around 1.5 mSv/year.
Collective dose is measured in "man-sieverts" (man.Sv)