When given a certain amount of radioactive material, it is customary to refer to the quantity based on its activity rather than its mass. The activity is simply the number of disintegrations or transformations the quantity of material undergoes in a given period of time.
The two most common units of activity are the Curie and the Becquerel. The Curie is named after Pierre Curie for his and his wife Marie's discovery of radium. One Curie is equal to 3.7x1010 disintegrations per second. A newer unit of activity if the Becquerel named for Henry Becquerel who is credited with the discovery of radioactivity. One Becquerel is equal to one disintegration per second.
It is obvious that the Curie is a very large amount of activity and the Becquerel is a very small amount. To make discussion of common amounts of radioactivity more convenient, we often talk in terms of milli and microCuries or kilo and MegaBecquerels.
Radiation is often measured in one of these three units, depending on what is being measured and why. In international units, these would be Coulombs/kg for roentgen, Grays for rads and Seiverts for rem.
Pictures of the Curies and of Becquerel.