An ABSTRACT is the short paragraph(s) found before the journal article begins. Authors use abstracts to provide a summary of the article so that readers will know the main idea before reading the entire article:
An ANNOTATION can be used to provide readers with a description/critique of the source so that they know whether or not the information in the article is useful to them. Annotated bibliographies should highlight strengths and weaknesses of sources as they relate to each other.
Still not sure you can tell the difference?
Try thinking about it this way:
When a new car rolls off the line, the company that created that car will develop all kinds of advertisements and brochures explaining all of the wonderful features that their car offers. These publications are great ways to find out what the makers of the product think about what they have been able to create. Information published by the auto maker is similar to an abstract that is written by the author or publisher.
Months later, outside companies like Edmunds or Consumer Reports will put this same vehicle through a series of tests to evaluate its performance. Usually they will try out several cars of the same period in the same vehicle class (ex: Dodge Charger vs Ford Mustang). Those companies will then publish a report that discusses the pros and cons associated with each individual model and then compare all similar models to each other to determine which are among the best and the less than best for that particular study. This is what you are doing when you are building an annotated bibliography. You are putting your sources through a sort of test to determine what is great about the source and then to examine where some of its potential pitfalls lie. Having this knowledge readily available when you begin writing your research can be immensely helpful. This is why entire books worth of annotations are published every year!