An annotated bibliography is essentially a bibliography, or a list of citations, where each citation is followed by an annotation, or a short summary of the work and an analysis or evaluation of the content. 

The format should follow the rules of a specific citation style such as APA or MLA, often with the citation using a hanging indent and the annotation on a new line below each entry. They should be double spaced, without extra spaces added between citations and paragraphs.

In APA, the entire annotation paragraph is indented 0.5" from the left. Do not indent the first line of the paragraph. If there are additional paragraphs, indent the first line of each additional paragraph another 0.5".

In MLA, indent the entire annotation 1" from the left. If you have additional paragraphs, indent the first line of each one another 0.5".

An annotation is generally no longer than a paragraph (100-150 words), but consult your assignment and instructor for preferred requirements. The summary part may include information such as "What is the source about? What are the main arguments?" while the evaluation part may include information such as "Is this source useful to you? How does it compare with other sources? How does it shape your argument?"

You can find examples on the APA Annotated Bibliographies and MLA Annotated Bibliographies pages.