A bibliography is a list of sources that you have consulted.
An annotation is a description or analysis.
Putting them together, an annotated bibliography is a list of citations with a brief description following each source.
What Should be in an Annotation?
What's in an annotation?
An annotation consists of two parts: a brief summary of the source (usually no more than a sentence or two), and an analysis of the source. Talk about the author's credentials and POV. Why is this source relevant for your work? What are its strengths and weaknesses?
What elements should go in an analysis?
Try thinking about the topics and questions on the Evaluation of Sources tab and putting that information in your analysis. You don't have to answer all of the questions there, but it's a good start.
How long should an annotation be?
It depends! The best thing to do is check with your professor for specific requirements. In general, your annotation should be clear and concise, but still give enough detail for the annotation to be useful to a reader.