I cant find a good answer on google?

"Ryan Gaffuri" <rgaffuri (AT) cox (DOT) net> wrote

A set does not have meaningful duplicates while a multi-set does. Thus the
following are equivalent sets but different multi-sets: {1}, {1,1}

A multi-set or bag is a data structure which allows duplicate
elements. A set is a data structure which does not allows duplicate
elements. Neither has any ordering. Multi-sets have modified versions
of standard set operations -- Union, Intersection, difference,
subsets, etc.