An annotated bibliography is an alphabetically organized reference list of sources that have been reviewed for a particular topic that also includes brief evaluative descriptions of each of the sources. Note, the operative word in this discussion is "evaluative." Bibliographies may very well include abstracts, or article summaries, for the materials included, but an annotation has a different focus. The annotation may provide a descriptive overview of the source being covered, but it also includes a critical assessment of the source as part of the description.
In other words, if a researcher were evaluating recent journal articles that update research on the cloning of animals for inclusion in an annotated reference list, he or she would go beyond merely summarizing article content and might examine and comment on the usefulness of the sources, comment on the reliability of the authors of the articles covered, evaluate the scope of the articles, and examine the articles' contributions to existing literature. The extent of the information included in the annotation would depend on the purpose of the bibliography and the intended audience. A highly critical, scientific audience would expect considerable detail in an annotation. A college professor might allow for less detail for a practice assignment for the purpose of teaching annotation.