A descriptive annotation provides a full citation of the work, as well as a brief overview or summary, without evaluating the work.
A descriptive annotation may summarize:
the main purpose or idea of the work
the contents
the author's conclusions
the intended audience
the author's research methods
Example in MLA style:
London, Herbert. "Five Myths of the Television Age." Television Quarterly, vol. 10, no. 1, Mar. 1982, pp. 81-69.
Herbert London, the Dean of Journalism at New York University and author of several books and articles, explains how television contradicts five commonly believed ideas. He uses specific examples of events seen on television, such as the assassination of John Kennedy, to illustrate his points. His examples have been selected to contradict such truisms as: "seeing is believing"; "a picture is worth a thousand words"; and "satisfaction is its own reward." London uses logical arguments to support his ideas which are his personal opinion. He doesn't refer to any previous works on the topic. London's style and vocabulary would make the article of interest to any reader.
A critical annotation includes a full citation of the work, as well as value judgements or comments (positive and/or negative) on its effectiveness.
A critical annotation may discuss some of the following features:
the importance of the source compared to other writing on the subject
the author's bias or tone
the author's qualifications for writing the work
the accuracy of the information
Example in MLA style:
London, Herbert. "Five Myths of the Television Age." Television Quarterly, vol. 10, no. 1, Mar. 1982, pp. 81-69.
Herbert London, the Dean of Journalism at New York University and author of several books and articles, explains how television contradicts five commonly believed ideas. He uses specific examples of events seen on television, such as the assassination of John Kennedy, to illustrate his points. His examples have been selected to contradict such truisms as: "seeing is believing"; "a picture is worth a thousand words"; and "satisfaction is its own reward." London uses logical arguments to support his ideas which are his personal opinion. He doesn't refer to any previous works on the topic; however, for a different point of view, one should refer to Joseph Patterson's, "Television is Truth" (The Journal of Television 45 (6) November/December 1995: 120-135).London's style and vocabulary would make the article of interest to any reader. The article clearly illustrates London's points, but does not explore their implications, leaving the reader with many unanswered questions.
This example, and more, can be found here: Memorial University "How to Write Annotated Bibliographies"