Assimilation is a process in which formerly distinct and separate groups come to share a common culture and merge together socially.  This would be very similar to around one hundred of college football players coming together from all over the state or country to share one common culture as a team.  Pluralism exists when groups maintain their individual identities.  This concept is seen within the United Nations where countries have come together, but they maintain their individual identities.  For example, France is not called the United Nations; they are called the Country of France or the French Delegate to the U.N.