A collection can include different types of elements. Whereas a set is a well-defined collection of distinct elements and it is enclosed in “{}.”

E.g.:

The collection of good teachers is a collection but not a set

Whereas when we take all the teachers of the school, then it is set. Because when we take all the teachers, then it is a definite number, so it called a set. But when we say, good teachers, it changes opinion wise, so we can say it is not definite hence we can call it a collection as it is not well defined.