An annotated bibliography is a list of citations to books, journals, articles, documents, websites and periodicals that also contains explanations from the bibliographer about each source cited. The descriptive explanations, or comments, are concise.
While brevity is encouraged, the length of the explanations isn't dictated by a firm word count, but rather should be as long as necessary to put across the bibliographers' thoughts on the material.
The annotation can simply summarize, but it generally goes further and is evaluative. The bibliographer will compare one source to another with the goal of honestly enlightening the reader about the accuracy, relevance and quality of the sources cited.