From a psychosocial standpoint, how is violence different from aggression?
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.