An annotation summarizes the essential ideas contained in a document, reporting the author's thesis and main points as well as how they relate to your own ideas or thesis. There are two types of annotations: summative and evaluative (see examples under the 'Types of Annotations' tab on this guide). Annotations are typically brief (one paragraph) but may be longer depending on the requirements of your assignment.
If you are creating an annotated bibliography for a class assignment, check with your instructor to determine the citation format, length and the type of annotations you will be writing.