An annotated bibliography is a list of citations to books, articles, and other sources. Each citation is followed by a brief (usually about 150 words) descriptive and evaluative paragraph, the annotation. The purpose of the annotation is to inform the reader of the relevance, accuracy, and quality of the sources cited.
You will summarize the relevant information you're getting from that source (but remember, this isn't your paper – you are summarizing, not presenting all the information itself) as well as how this well help you in your research (by providing background information, by exploring a certain angle, by presenting a contrary idea...).
Abstracts are the purely descriptive summaries often found at the beginning of scholarly journal articles or in periodical indexes. Annotations are descriptive and critical; they expose the author's point of view and authority.
Creating an annotated bibliography is not simply an academic exercise. An annotated bibliography is a tool to help you summarize your source content and evaluate its place within your research. If a source satisfies your "usefulness" criteria, that source belongs on your annotated bibliography. Of course, evaluation of sources goes beyond "usefulness" to the other elements of authority, credibility, currency, and purpose.
This process begins the transition from reading sources to incorporating content (ideas, quotes, paraphrasing) into your work. It is time to "make sense" of the knowledge you have gained from your research. This knowledge is the foundation on which to build your own voice, explain your methodology, discuss your conclusions, and report on your new knowledge.
Crafting an annotated bibliography calls for the application of a variety of intellectual skills: concise exposition, succinct analysis, and informed library research.