Purdue University's Online Writing Lab, OWL, has a good section on understanding Annotated Bibliographies:
It includes definitions, examples and samples for the MLA, APA and Chicago styles.
Click here for example annotations in MLA, APA, & Chicago styles.
After reviewing your assignment, these are possibly additional factors to consider when annotating a source:
(from Joanne M. Burkhardt's 35 Practical, Standards-based Exercises for College Students)
To write an annotation, you will comment, in paragraph form, on the following elements: