Annotated Bibliographies are important research tools that can be used to build context around a selected set of sources and show where your research fits amongst the larger body of knowledge on a topic. While peer-reviewed journals often make up the bulk of an annotated bibliography's content, these documents can include a range of sources including books, news articles, conference proceedings, and government reports. Whereas "Works Cited" lists are organized alphabetically, annotated bibliographies can be organized in a variety of ways such as theme, author, date range, or topic.
Think of annotations as providing a succinct summary of a source coupled with a succinct analysis. Well-constructed annotations summarize a source's content and arguments, evaluate a source's usefulness and reliability, compare a source to other published literature on the topic, and establish how the source may be useful for future research.
While annotations are typically a paragraph in length (approx. 150 words), they can range from a couple of sentences to multiple paragraphs spanning several pages. Don't focus on length; focus on content and the dialogued are you are starting around your selected source.