A summary is a shortened version of someone else's writing or thoughts.

Summaries vary in length and amount of details depending on a teacher's requirements, the length of the original source (article, book, passage) and the purpose of the summary; however, all summaries must:

•	be shorter than the original source (article, book, passage) approximately one third the length of the original source; 
•	include the main idea of the original source in your own words; 
•	include major details (also known as supporting ideas) in your own words; 
•	should not include your knowledge, ideas or opinion unless your teacher requests it. 
•	identify the author, title of article, title of publication, where published, publisher, year of publication, and page information, at the top of the page of your summary (or in-text.) 

Taken from:
http://depts.gallaudet.edu/englishworks/writing/main/summary.htm