From a psychosocial standpoint, how is violence different from aggression? I recently watch Steven Pinker's video on violence and figured out that I don't understand the difference between violence and aggression.

According to Wikipedia aggression is:


  Aggression, in its broadest sense, is behavior, or a disposition, 
      that is forceful, hostile or attacking.


and for Violence they use the WHO's definition:


  the intentional use of physical force or power, threatened or actual, 
  against oneself, another person, or against a group or community, 
  which either results in or has a high likelihood of resulting in injury, 
  death, psychological harm, maldevelopment, or deprivation


At first sight it seemed that the difference could be intentionality, i.e., aggression has intentionality attached to it and violence doesn't.

But the WHO's report linked says:


  The definition used by the WHO associates intentionality with the committing of the act itself, irrespective of the outcome it produces


So the difference is not all that clear to me.
The definition of aggression and violence is a thorny issue, and there is considerable disagreement about the terms between disciplines, but also within disciplines. Sometimes authors use the terms interchangeably. 

In psychology, the term aggression is often used as the broader term that encompasses all forms of harmful behavior.

For example, according to the widely cited definition by Baron and Richardson (1994), "aggression is any form of behavior directed toward the goal of harming or injuring another living being who is motivated to avoid such treatment". This would include verbal aggression, or indirect forms of aggression such as gossiping.

In contrast, the term violence is often limited to extreme forms of physical aggression.

For example, Anderson (2000) defines violence in the Encyclopedia of Psychology as 


  a subtype of aggression, generally used to denote extreme forms of aggression such as murder, rape, and assault. All violence is aggression, but many forms of aggression are not violent. 


References

Baron, R. A., &amp; Richardson, D. (1994). Human aggression. New York: Plenum Press. 

Anderson (2000). Aggression and violence. In A.E. Kazdin (Ed.) Encyclopedia of psychology, 8, 162-169. New York &amp; Washington D.C.: Oxford University Press and the American Psychological Association.