Abstracts are the purely descriptive summaries often found at the beginning of scholarly journal articles or in periodical indexes. Annotations are descriptive and critical; they may describe the author's point of view, authority, or clarity and appropriateness of expression.
Check with your instructor to determine what they want to be included in annotations. In general, an annotation should inform the reader of the quality and relevance of the source. Annotations are detailed but succinct, typically about 150 words, and include:
Taken from http://libraryguides.library.clark.edu/annotated-bibliography