Radioactivity is the amount of radiation emitted. There are three types of radioactivity, alpha, beta and gamma.
Alpha radioactivity is caused by helium atoms, travels very short distance and stopped by almost anything. Beta radioactivity is caused by electrons and travels longer than alpha radiation and stopped by aliminium foil etc. Gamma travels the longest and can be stopped by a thick concrete.
For other radiation unit conversions, please go to Radiation Unit Conversion