The purpose of the annotated bibliography is to inform the reader of the relevance, accuracy, and quality of the sources cited. Annotated references (or bibliographies) are different from abstracts in that abstracts are purely descriptive summaries of a research paper found at the beginning of articles. Annotated bibliographies are descriptive but also critical; they should summarize the theory/hypotheses/method/findings of the study, evaluate the validity or contributions of the study, and explain how the work illuminates your own topic of study.
A guide to preparing an annotated bibliography from Cornell University Library.