MAT. TYPE , MAT. GROUP , MAT. CLASS ?

Dear All,

Can ne one explain me about the diff. between mat type , mat. group & mat class -- if i do not give mat type while creating a material where would i stuck OR if i do not give mat group while creating a material where would i stuck ,OR if i do not give mat class while creating a material where would i stuck .? where will the process stops in SAP .

Regards,

sap11