It is always short. It is written for the same audience as the article, so it uses the same level of technical language. It always summarizes the major points of the results. It ordinarily summarizes the major points of the materials and methods, and of the discussion. In most disciplines, it never includes bibliographic citations.
Abstracts are also usual for review articles.
The more familiar you are with the contents of an article, the easier it is to write an abstract. If you wrote the article yourself, you obviously know what is in it, but professional abstract writers routinely make abstracts of articles they haven't written, and you can, too. The first step is to identify the major point or points of the article. Sometimes it helps to make an outline , but that is not always necessary. When you have written down the main points, then look to see what information is crucial to lead up to those points.