The Sievert (like the older rad) is a biological unit. It takes into account how vulnerable different parts of your body are and it can be used to determine long term cancer risk.

The Rontgen is a physical measure. It says how strongly radiation affects a unit of air. Since you're not air Rontgens are a pretty rough measure. The superior physical measure is the Gray (similar to the older rem) which counts how much energy per unit mass of tissue is absorbed.

Other units count how many "bullets" per second of radiation are released overall, these being Curies and Becquereles - while paying no attention to the kind of bullets that are counted.