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. 

• 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

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

Each annotation starts with a full citation in MLA, APA, or Chicago style. 

Each annotation 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)

You might also include an assessment of the source:

• 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