A bibliography is a simple list of the sources used in your research, while 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 source, but it also provides an evaluation of its 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 your opinion of the relevance, accuracy, and quality of the cited source.
An annotated bibliography 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.