This short video from the Lincoln Memorial University Library explains the what, why, and how of an annotated bibliography.
According to the Purdue Online Writing Lab (OWL):
A bibliography is a list of sources (books, journals, Web sites, periodicals, etc.) one has used for researching a topic. Bibliographies are sometimes called "References" or "Works Cited" depending on the style format you are using. A bibliography usually just includes the bibliographic information (i.e., the author, title, publisher, etc.).
An annotation is a summary and/or evaluation. Therefore, an annotated bibliography includes a summary and/or evaluation of each of the sources.
According to EasyBib:
Your annotations should include either the main points from the source, as well as the topics covered, the approach used and any findings, or your critical evaluation. A standard annotation is 150-200 words or approximately one paragraph. Take care not to include any unnecessary details, as the goal is to summarize each source as succinctly as possible and, in some cases, evaluate them.