A bibliography is a list of sources that you have consulted. An annotation is a description and analysis. So therefore, an annotated bibliography is a list of citations with a brief description of each source.
Q: What's in an annotation?
A: An annotation consists of two parts. The first part is a brief summary of the source, usually no more than a few sentences. The second part is an analysis of the source. Analyze the author's credentials and point of view. How relevant is the source? How accurate? What are the strengths and weaknesses of the source?
Q: What elements should go in an analysis?
A: Go to the Evaluate Information tab and answer the questions about your source.
Q: How long should an annotation be?
A; Typically an annotation is about 150 words. You want to be concise, but still give enough detail for the annotation to be useful to your reader. But your professor will let you know the specifics for your assignment.
Here are some websites with good examples of annotations that you can follow.