An annotated bibliography is a list of citations to sources, such as books and articles. Each citation is followed by an annotation, a brief descriptive and evaluative paragraph, about 150 words long, that analyzes the source. An annotated bibliography usually looks like any other bibliography with alphabetized citations of sources, except that here each source is followed by an explanatory paragraph. This work can form the basis of a literature review later in the writing process. The purpose of the annotation is to inform on the relevance, accuracy, and quality of the sources cited.
An annotation is not only a summary of the source in question. It should be a short but critical analysis as to why and how the source fits into the larger research question. An abstract functions as a summary, an annotation should be contextual to the specific topic at hand. It should be both descriptive and evaluative.
What about formatting?
Most of the major citation styles call for a hanging first line on annotated bibliographies. This means the first line of the citation will align with the left margin of the page, and all subsequent lines of the citation and annotation will indent to the right.
Patra, A., T. Park, M. Kim, and Z. Yu. 2017. "Rumen Methanogens and Mitigation Of Methane Emission by Anti-Methanogenic Compounds and Substances." Journal of Animal Science and Biotechnology 8:13.
This study reviews some of the work to date (2017) identifying ruminal methanogens and the in vivo and in vitro effects of anti-methanogenic compounds. Of specific interest is the summary of evidence suggesting that archaea make up only ~10% of the ruminal microbiome (see "Overview of methanogens present in the rumen"). Also of note, this paper cites work indicating many rumen ciliate protozoa have ecto- and endo-associated methanogenic archaea (see "Methanogens associated with rumen protozoa"). However, most ruminal methanogens are "free-living" (i.e. not protozoa-associated; see "Free-living ruminal methanogens").