is short list of the different types of units:
http://www.mcgill.ca/ehs/radiation/basics/units/There are 2 basic areas of confusion.
1 - Old units (names) vs new.
Just like 'inch/foot/yard/mile' are the old unit (name) of length measure and 'meter' is the new International Standard ('S.I.')
unit of measure radiation has old units and new.
Old unit : New (S.I.) unit
Curie > Becquerel
Rad > Gray
Rem > Sievert
The webpage listed above shows the conversion factors from the old unit to the S.I. unit.
2 - Different measures of radiation.
There are different types of radiation (alpha, beta, gamma, x-rays, neutron) and there
are different ways to measure radiation:
Activity - how fast the radioactive substance is disintegrating:
Units: Curie or Becquerel
Energy - how much energy the radioactivity is imparting to an area:
Units: Rad or Gray
Biological effect - different types of radiation have different effects on biological tissue.
Units: Rems or Sieverts
Note the for the S.I. units prefixes before the name can be used. The common prefixes that are being used
are milli (one thousandth, 'm') and micro (one millionth, 'u').
A milliSievert is one thousand times smaller than a Sievert, and a microSievert is one million times smaller than a Sievert.
A microSievert is one thousand times smaller than a milliSievert, or put in the reverse a milliSievert is one thousand
times larger than a microSievert.