Put simply, an annotated bibliography is a list of citations for resources and materials consulted for a research project. The resource is cited, and the citation is accompanied by a brief description, or annotation, of the resource. The description may also include a very short analysis or evaluation of the resource in question, stating the resource's validity, authority, and quality. As such, annotated bibliographies generally lend a critical or analytical aspect to a research project.
Annotated bibliography as precursor to the literature review
As a list of sources and materials, the annotated bibliography lays out resources consulted in the formation of an argument or thesis. It doesn't actually make the argument itself, or offer an overview or understanding of a particular subject or topic. That's the job of the literature review. What's more, the annotated bibliograph looks like a list of citations with paragraphs attached. The literature review is a prose document with an introduction, a body, and a conclusion.
All the same, the annotated bibliography is a valuable tool in the writing of the literature review, as it describes and briefly evaluates each resource's value and use. A good annotated bibliography entry should include the following:
A brief and concise summary of what the resource is about.
An examination of the credentials of the author: who she is, what her expertise is, and so forth.
A review of the strengths and weaknesses of the resource: what's valuable to the argument you're making, what's detrimental or missing, and so forth.