A bibliography is a list of sources (books, journals, Web sites, periodicals, etc.) one has used for researching a topic. Bibliographies are sometimes called "References" or "Works Cited" depending on the style format you are using.
A bibliography usually just includes the bibliographic information (i.e., the author, title, publisher, etc.). An annotation is a summary and/or evaluation. An annotated bibliography, therefore, includes a summary and/or evaluation of each of the sources. Your annotations should give your instructor a clear idea as to why you've used the sources you have found for your assignment.
Elements of a good Annotated Bibliography:
1) Sources cited according to the appropriate citation style (MLA, APA, CBE/CSE, etc.).
2) Explanation of main points and/or purpose of the work. This will show your instructor that you have read and thoroughly understand the source. DO NOT copy and paste the abstract. Your annotations come from YOUR understanding of the source being evaluated and how it fits in with your project/assignment.
3) Verification or critique of the authority or qualifications of the author. You should be able to determine whether or not the source comes from a reputable journal (one that has a blind peer review process for evaluating inclusion), or is a book from a reputable academic publishing house. You want to answer the question: "Why is this source good enough for inclusion in my bibliography?"
4) Comments on the worth, effectiveness, and usefulness of the work in terms of both the topic being researched and/or your own research project. This is where you answer the question: "Why is this a useful source of information for my assignment?"
Lastly, your instructor may want for you to evaluate the quality of the research itself. This would mean answering questions about potential bias, types of methods used (and not used), and whether or not the study answers its hypothesis sufficiently.
Source: UNC Writing Center - Annotated Bibliography