An annotated bibliography provides information about each source you have used
One of the reasons behind citing sources and compiling a general bibliography is so that you can prove you have done some valid research to back up your argument and claims. Annotated bibliographies include additional details that provide an overview of the sources themselves.
Each annotation provides essential details about a source
Readers, researchers or instructors reading an annotated bibliography will get a snapshot of the important details that they need to know about each source. As a researcher, you have become an expert on your topic: you have the ability to explain the content of your sources, assess their usefulness, and share this information with others who may be less familiar with them.
Some types of annotations provide details about what the paper's author has done with or thinks about each source
Certain types of annotations (Evaluative Annotations, for example) may provide a brief analysis of the source by the paper's author, including details about what information is most important or not, how it the source fits into the broader scope of the paper and why it may or may not be useful to others.