An annotation is a brief piece of writing about a longer written source, such as a journal article or a work of fiction. An annotation minimally contains a brief summary, but may also include an assessment or reflection about the work being annotated. The length of an annotation can vary from a few sentences to many paragraphs, depending on the need.
An annotated bibliography is much like a Works Cited page, except each entry also contains a brief summary. You'll want to gather your sources, create a Works Cited list in alphabetical order, then under each citation provide an annotation of that source.
Purdue's Online Writing Lab does a very good job of describing the Annotated Bibliography. Read about it here.