Research has shown these four zones exist for most North American people:
The intimate zone for lovers and those dear to us is 1.5 feet (45 cm.) or less. [Europeans: 20-30 cms (8-12inches); Australians: 46-122 cms (18-49 inches); Japanese: 25 cms (10-11 inches)] Interestingly, at this close range not only body language is meaningful but also our unconscious sense of smell can reveal much information about the partner's mood and health. See more on the importance of pheromones in relationships and body language here.
The social zone from 1.5 feet (45 cm...) to 4 feet (120 cm...) is where we comfortably converse with friends. Interestingly, this distance is where body language can be observed very effectively.
The social zone where conversation is conducted with casual acquaintances is no closer than 4 feet (120 cm..) and out to 7 feet (360 cm..). This zone is at least two arm lengths away and therefore out of striking distance. This unconscious habit of staying a safe striking distance away from those who are not our intimate friends probably is something left over in our DNA from our ancestors millions of years ago.
The public zone where strangers and authority figures are most comfortably acknowledged is beyond about 7 feet (360 cm..). This longer distance may say something about how we unconsciously fear strangers and authority figures.