A summary expresses only the main points of a source or part of a source. It is a brief, condensed version of the ideas in the source. You might summarize if you want to show the primary point of one section of an article or provide a general overview of the source material. Here is one example of a very brief summary which shows the main point of one section of the following source:
Fussell, Elizabeth. "Constructing New Orleans, Constructing Race: A Population History of New Orleans." Journal of American History 94.3 (2007): 846–855. Print.
From its founding in the early 1700s, New Orleans has been a multicultural city, but the racial and cultural composition of its population has shifted and changed a great deal over time. Slaves and former slaves, European colonists and immigrants, whites, Creoles, Haitians and African-Americans—all can claim an important part in the city's history.
A longer summary might consist of six to ten sentences or more, perhaps with the purpose of presenting an overview of a source or detailed background material relevant to your paper. For example, this summary covers an author's very long description of the changes in the cultural make-up of New Orleans in the past 250 years:
Since the early 1700s, the population of New Orleans has experienced racial and cultural shifts that have been caused primarily by social and economic factors. At its French founding, New Orleans was a city inhabited by the slaves who essentially built it. It grew for two centuries, leading up to and through its acquisition by the U.S., as a population largely made up of descendants of slaves; white French colonists, Creoles and ex-slaves from Haiti; and European immigrants who moved to New Orleans in search of work. Growth slowed in the twentieth century, when the Jim Crow era resulted in division of New Orleans into segregated white and black societies, and large-scale flight of the white population to the suburbs left the city with a black majority from the 1960s through most of the 1980s. In its most recent stage—since Hurricane Katrina in 2005—the city first emptied, then became repopulated with white and black former residents and new Latino migrant workers (Fussell 847, 850–853).