If a substances is Capable of always emitting radiation under any conditions we say it is radioactive. there are three types of radiation which are Alpha, Beta and Gamma. These types of radiation are always giving of by rocks, food , building materials , air and cosmic rays around us this is called background radiation . each type is capable of penetrating different materials.
when radiation collides with neutral atoms or molecules it alters their structure by knocking of electron. This will leave behind ions this is called ionising radiation.