An annotated bibliography is a list of sources (a bibliography) with a short description of what that source is and how it relates to other research on the topic.
Annotated bibliographies are a great way to organize your thoughts on research and topics in preparation for writing a paper. They can also be used to summarize the state of research on a topic. While annotated bibliographies are not a major source, they do get published and are considered gray literature. They can also be used in informal reports when your goal is to bring someone else up to speed quickly on research and give them enough information to decide when articles to read further.
Example of an annotation from a published annotated bibliography:
Wyss, M., 1986, Regular intervals between Hawaiian earthquakes: implications for predicting the next event: Science, v. 234, pp. 726-728.
This paper notes that mainshocks in the Kaoiki area west of Kilauea Caldera experienced magnitude 5.5-6.6 earthquakes at intervals of 10.5 +/- 1.5 years during 1941-1983. This is very useful for understanding Hawaii tectonics and for intermediate term earthquake prediction in this area. The fact that moderate earthquakes can be expected to recur in this area is relevant to hazard analysis. The hypothesis of this paper could be used to calculate how the probability of strong ground shaking changes on Kiauea with time. Hazards calculated over a 50 year period would include several moderate earthquakes at Kaoiki, however.
from: Klein, F.W., 1994, Annotated bibliography, seismicity of and near the island of Hawaii and seismic hazard analysis of the east rift of Kilauea: U.S. Geological Survey Open-File Report 93-551. 11 p., doi:10.3133/ofr93551B
Remember that the annotation is short, you are not writing a full report on the source, but you do need to provide enough information to demonstrate that you read and understood the source. You should plan on a minimum 5-6 sentences for each annotation. This means you need to make every word and sentence count. The annotations that you write will have two components: descriptive and analytical.
The descriptive part of the annotation will be the first part of the annotation, usually 2-3 sentences. In this part of the annotation, you should describe the main focus of the source. If the source is reporting the results of the study, summarize the methods used and results of that study.
The analytical section of your annotation should be a bit longer. In this part, you want to talk about how the source contributes to your research topic. Does it relate to other sources in the bibliography? If the source is reporting on a research study, what were some of the limitations of the study? How might that study be built upon in the future?