Whereas a bibliography is a simple list of the sources used in your research, an annotated bibliography is a list of those sources plus a brief (typically around 150 words) summary and evaluation of each source. The annotation not only describes the content of the item but also provides the reader with an evaluation of the item's methods and/or conclusions and its usefulness to your research. Unlike an abstract (which is a simple summary), an annotation is descriptive and critical; it exposes the author's opinion of the relevance, accuracy, and quality of the cited source.
There are two benefits of an annotated bibliography. The first is that it compels you, the researcher, to better understand your own research. In order to create the annotations, you have to thoroughly review your sources and be able to explain and form conclusions about them based upon your overall knowledge of the topic. Secondly, your annotations will help readers decide if a source will be useful to them in their own research.