An abstract is a summary of a longer written document. It is written with the same organization as the original document, usually without “review” or comment. Abstract lengths vary by discipline and purpose of the project; generally, they are 50-100 words for a short paper and 150-200 for a longer one. Abstracts are typically placed prior to the manuscript. Abstracts fall into two categories: indicative and informative.
The indicative abstract covers a less structured document (like a book, essay or editorial). It helps readers to understand the focus, arguments and conclusions of the larger document so that they can determine whether to read it more thoroughly.
An informative abstract is used for more strictly structured documents (like scientific experiments or investigations) and includes the elements of the original research report: its objective, methods, results, and conclusions.
Whichever type of abstract you write, it should be able to stand alone as a statement separate from the larger document.