A sievert is a measure of the response of the body to radiation.
It's measured in J/kg.
The initial dose is measured in grays. They are weighed depending on the type of radiation and the type of tissue. Since α is the most damaging, it has a weighting of about 20 whereas γ, which mostly just passes through us, has a weighting of 1. Gonads, breast and thyroid tissue, for example, are more likely to have a negative effect than, say, bone so different weighting for different tissues.
Receiving 1 Sv over a lifetime gives about a 5% chance of getting a fatal cancer. In the UK, the radiation we get from rocks, cosmic rays and the carbon and potassium in our bodies, we receive about 2.5 mSv per year. Of course, if we have medical x-rays, we get more.
http://www.sixtysymbols.com/videos/radiation.htm
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