An annotated bibliography is a list of citations to books, articles, and documents that follows the appropriate style format for the discipline (MLA, APA, Chicago, etc). Each citation is followed by a brief (usually about 150 word) descriptive and evaluative paragraph -- the annotation. Unlike abstracts, which are purely descriptive summaries often found at the beginning of scholarly journal articles or in periodical indexes, annotations are descriptive and critical.
The purpose of the annotation is to inform the reader of the relevance, accuracy, and quality of the sources cited. The annotation exposes the author's point of view, clarity and appropriateness of expression, and authority.