Usually, when you write a paper, you 1) find your sources, 2) evaluate their quality, 3) use them to write an essay, and then 4) add a bibliography of sources used to the end of your paper. An annotated bibliography essentially skips step 3), so you find, evaluate, and cite your sources but you don't write the paper.
See the different sections of this guide for more information on each step.
Purdue's Online Writing Lab (OWL) has some great examples in both APA and MLA format.
Below is an example from OWL: note how it begins with a single citation to Anne Lamott's "Bird by Bird" and continues with a couple paragraphs. The first paragraph summarizes the content of the source while the second paragraph analyzes the source.