Annotated bibliographies are lists of sources, with a brief description of the source below each item. Basically, it is a reference list with a descriptive paragraph below each source.
Creating an annotation helps you reflect on the relevance, accuracy, and quality of the sources you are citing. Annotations may seem tedious, but done well, will further your thinking on your topic and are extremely useful in writing your literature review; you've already set out connections (or lack of) among your topics.
An annotation is not an abstract. From Cornell Library:
"Abstracts are the purely descriptive summaries often found at the beginning of scholarly journal articles or in periodical indexes. Annotations are descriptive and critical; they may describe the author's point of view, authority, or clarity and appropriateness of expression."
This should be the source formatted in the appropriate style like APA or MLA. It should look identical to sources found in a references list at the end of a paper. The list of references should be alphabetical order.
This is a short paragraph (or two), describing, analyzing, and critiquing each source. The annotation goes directly below the reference. The annotation should be more in-depth than just a summary. There should be an analysis of the source that goes along with the description.
The annotations shouldn't just describe what the source is about, but also how it adds evidence to your specific research question. What is unique about the information presented in this source that will help build an argument? There should also be an evaluative description of the source that discusses the appropriateness of the source. It should answer questions like currency, author credibility, suitability of source type, types of evidence presented, and source objectivity.