Dear Wahiba
topic is well explained in online help and discussed oftene here. If you check document e.g. SAP EHS Management for Beginners
you will find some additional links etc.
First: don't mix the terms
Second: pay attention in link to "material" (this can be a "on top" complexity to understand)
A material can be linked to a REAL_SUB and it can be "hazardous" (according regulations) but need not and it can be "dangeorus" (According/regulations); but need not.
C.B: