An annotated bibliography gives an account of the research you have done on a given topic.  The purpose is to inform the reader of the relevance, accuracy, and quality of the sources cited. 

An annotated bibliography is NOT the same as an abstract, which simply summarizes a work's main points.  

• To define the scope and quality of one's own research
• To review the literature published on a particular topic
• Provide a historical context for your research

• Description can include
• the main point, content or argument of a source. 
• the author's point of view
• any shortcomings or bias in the author's argument, research.
• the author's credentials
• any significant special features of the work (e.g., glossary, appendices, particularly good index)

• Assessment
• What is the value of this source to your research? 
• How will you use it in your research?
• Your own brief impression of the work