An annotated bibliography is an organized list of your sources that includes additional information about the sources. This information can be descriptive, explanatory, and/or evaluative in terms of quality, relevance to your own research project, as well as explorative in how the work provides options for further research.
To write a good annotated bibliography you need to be:
Concise: Go to the point of what is the book/article about, in few words, summarize.
Evaluative: Determine who is the author, what is his/her expertise in the topic, how reliable is the information
Critical: Reflect on what is the strength and weakness of the work, what is missing, etc.
Comparative: how each piece review compares to other similar works
Useful site with tips on how to write an Annotated Bibliography: