An annotated bibliography lists citations to sources used in the research paper. Each citation contains a short annotation, usually about 150 words, which describes and provides evaluative content about the work so the reader can judge the relevance and quality of the work cited.
An annotation is slightly different from an abstract. An abstract is merely a summary of the work whereas an annotation provides more descriptive and critical analysis of the work. An annotated bibliography may contain all or most of the following elements: a brief description of the work, the scope and purpose of the work, the intended audience, relevance or usefulness, helpful features of the work, observations of the author, comments of the reader.
Here's an example of an annotated bibliography from Purdue's Online Writing Lab:
Lamott, Anne. Bird by Bird: Some Instructions on Writing and Life. New York: Anchor Books, 1995. Print.
In the sample annotation above, the writer includes three paragraphs: a summary, an evaluation of the text, and a reflection on its applicability to his/her own research, respectively.