An abstract is a short summary of your (published or unpublished) research paper, usually about a paragraph (c. 6-7 sentences, 150-250 words) long. A well-written abstract serves multiple purposes: an abstract lets readers get the gist or essence of your paper or article quickly, in order to decide whether to read the full paper.
What follows is a hypothetical example of a research paper based on an experiment. The experiment: Say you have just conducted the Milgram Study. Now you want to write the research paper for it. (Milgram actually waited two years before writing about his study.) Here's a shortened example of a research article that MIGHT have been written.