Radioactivity is the amount of radiation emitted. There are three types of
radioactivity, Alpha, Beta and Gamma.
Alpha radioactivity is caused 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.
Units are curie and becquerel.