An annotated bibliography is a bibliography [a list of the sources- like articles & books- referred to in a scholarly work] that gives a summary of each of the entries. The purpose of annotations is to provide the reader with a summary and an evaluation of the source. Each summary should be a concise exposition of the source's central idea(s) and give the reader a general idea of the source's content.
Summary: Some annotations merely summarize the source. What are the main arguments? What is the point of this book or article? What topics are covered? If someone asked what this article/book is about, what would you say? The length of your annotations will determine how detailed your summary is. Who is the intended audience?
Assessment: After summarizing a source, it may be helpful to evaluate it. Is it a useful source? How does it compare with other sources in your bibliography? Is the information reliable? Is this source biased or objective? What is the goal of this source?
Evaluation: Once you've summarized and assessed a source, you need to ask how it fits into your research. Was this source helpful to you? How does it help you shape your argument? How can you use this source in your research project? Has it changed how you think about your topic?
Information on the author(s): What is the author's authority? What are their qualifications?