A bibliography is a list of the sources that you used to research your topic (also known as Works Cited or References, depending on what format you're using). An annotation is a summary or explanation.
An annotated bibliography includes a summary/evaluation of each source.
The Purdue OWL suggests that your annotated bibliography should do one or more of the following:
Keep in mind that not all annotations must be the same length, and do not always have to assess and reflect. Sometimes just a brief summary is enough. Remember to check with your professor on what they expect for your specific assignment.