An annotated bibliography is a list of sources (books, articles, websites, etc.) with short paragraph about each source. An annotated bibliography is sometimes a useful step before drafting a research paper, or it can stand alone as an overview of the research available on a topic.
Each source in the annotated bibliography has a citation - the information a reader needs to find the original source, in a consistent format to make that easier. These consistent formats are called citation styles. The most common citation styles are MLA (Modern Language Association) for humanities, and APA (American Psychological Association) for social sciences.
Annotations are about 4 to 6 sentences long (roughly 150 words), and address:
Main focus or purpose of the work
Usefulness or relevance to your research topic
Special features of the work that were unique or helpful
Background and credibility of the author
Conclusions or observations reached by the author
Conclusions or observations reached by you
Annotations versus Abstracts
Many scholarly articles start with an abstract, which is the author's summary of the article to help you decide whether you should read the entire article. This abstract is not the same thing as an annotation. The annotation needs to be in your own words, to explain the relevance of the source to your particular assignment or research question.