A collection of objects which have one or more properties commonly is called a set.
Every object is having its own properties naturally. The properties of one object can be compared with the properties of another object. In some cases, the properties of objects may not be same but either one or more properties of two or more objects may be same in other cases.
If either one or more properties of two or more objects are same, the objects can be collected as a group by the well-defined properties and the group is called as a set in mathematics.
Basket Ball, Cricket Ball, Tennis Ball, Football, Pool Ball and Volley Ball are six objects and these six objects are used in various sports but these six objects are having two properties commonly. These two common properties made them to form a group.
All six balls have round shape. It is one common property in all of them.
All round shapes are not used as sports balls but these six balls are used in sports. It is another common property in them.
The shape and purpose are well-defined common properties of the objects and they made objects to belong to a group. This group of objects is called a set.