An Abstract and an Annotation are NOT the same thing!
An ABSTRACT usually precedes a journal article and explains, in a very concise way, what the article is going to discuss. The abstract is usually written by the author herself, and is intended to help researchers decide whether or not the work is relevant enough for them to read the entire article.
An example of an abstract - taken from an article found via Academic Search Complete - is shown below.
But isn't an annotation a short description of the work?
Yes, but annotations serve a different function from that of abstracts.
ANNOTATIONS are evaluations of a source written by an individual researching the given subject. An annotation will describe the utility of a work and the background of its author, but they also critique the strengths and weaknesses of the resource. To sum up: an abstract is an author written preview of their own work; an annotation is a description and evaluation of the work written by a third party.
Confused? Let's think about it this way:
VS.
MOVIE TRAILERS MOVIE REVIEWS
When a movie is released, the movie studio puts out a trailer (and some billboards) that advertise the movie to us. These trailers aim to give us a taste of what the movie is about in order to encourage us to go see the entire film. A movie trailer is the film world's version of an abstract.
Movies are also reviewed by columnists at the NY Times and other papers. These reviewers provide us with an overview of the film's plot, some background on its director and cast, and a critique of the film as a whole. As a movie review is a description and evaluation of a film, it is akin to an annotation.