A bibliography is a list of sources used for researching a topic. An annotation is a summary and/or evaluation. So an annotated bibliography is a list of sources that includes a summary and evaluation for each source. The amount of evaluation versus summary in an annotated bibliography will depend on the guidelines given by your instructor.
Why write an annotated bibliography?
There are two primary reasons to write an annotated bibliography: to learn about your topic and to help other researchers. Writing an annotated bibliography is excellent preparation for a research project, as you will not only be gathering a list of sources but critically evaluating them and noting your assessment. This process will assist you in developing and supporting a thesis later on. Extensive and scholarly annotated bibliographies are sometimes published, providing a comprehensive overview of everything important about a given topic. As a researcher, you may want to start a project by looking for an annotated bibliography that has been published on your topic.