A set is defined as: "A set is a finite or infinite collection of objects in which order has no significance, and multiplicity is generally also ignored (unlike a list or multiset)." (link)

 

This is my question: can it be a collection of real objects or does it have to be a collection of abstract or conceptual objects only?

As it is mathematics, the domain is really abstract. However, it is often the case that abstract ideas can be applied to nature and real world situations.

 

If I have a collection of fruit, lets say an orange, an apple and a banana, then I can describe this real world situation with a set, say [math]F = \{ O, A, B \}[/math] (with obvious notation). But this set is an abstract construction and the fruit physical objects.