An abstract is a short, and powerful statement that describes the research paper as a whole. The abstract should contain 150 to 250 words. The abstract provides a reason of selection and indexing. It allows the audience an overview to determine whether the paper is worth the reward of reading.
The two types of abstracts are descriptive and informative. The descriptive abstract indicates the information found in the paper, makes no judgment, and is very short with 100 words or less. The informative abstract explains the main arguments, includes all the parts of the paper, and the length varies. Abstract varies depending on the field the paper is covering, for an example, a history paper, and a psychology paper.
Look at the link below to et information on APA style and see an example of an research abstract in the sample paper.