Types of abstracts

1. Descriptive abstracts

• Tell readers what information the article contains
• Include the purpose, methods, and scope of the article
• Do not provide results, conclusions, or recommendations.
• Are always very short, usually under 100 words.
• Introduce the subject to readers, who must then read the article to find out the author's results, conclusions, or recommendation

2. Informative abstracts

• Communicate specific information from the article
• Include the purpose, methods, and scope of the article
• Provide the article's results, conclusions, and recommendations.
• Are short -- from a paragraph to a page or two, depending upon the length of the original work being abstracted. Usually informative abstracts are 10% or less of the length of the original piece.
• Allow readers to decide whether they want to read the article
  

Essential elements of the abstract are:

1. Background: A simple opening sentence or two placing the work in context
2. Aims: One or two sentences giving the purpose of the work.
3. Method(s): One or two sentences explaining what was done. (Described at length only if it is unusual)
4. Results: One or two sentences indicating the main findings. (Absolutely essential)
5. Conclusions: One sentence giving the most important consequence of the work. (Telling what the results mean).

Source: http://www.olemiss.edu/depts/writing_center/grabstract.html