An abstract should be 250 words or less. Its purpose is to allow other researchers to have quick overview of your paper or article, allowing for more efficient research. As you have done research in one database or another, you may have already noticed you could save time by reading over an abstract and deciding whether or not you should read the entire article. Your abstract is meant to do the same thing for others.
Abstracts should never be written in the future tense or "announcement" format, so "This paper will discuss..." is not an acceptable statement. Rather, you want to make assertions, for example,
Homeland Security is the result of the events of 9/11, leading to the creation of a new agency, new legislation, and heightened national security consciousness.
Be sure that all of the points to be made are contained within the abstract, and bear in mind that an abstract is one of the final steps in doing a paper of any sort because you will not know what you are encapsulating until you are done.