Interaction is the basic concept in sociology. When the action of one person stimulates a responce in another , it is called interaction. When there are two or more persons in interaction , to a concious of 'mind' , then it is called a group.
Human personality develops with in the group and man is made human by social interaction. Every group is characterised by certain common interest shared by all the members.
According ti Turner and Killian "a group always consists of people who are in interaction and whose interaction is effected by some sense that they constitute a unit".