An abstract is neither an introduction nor a conclusion. Although it must introduce the reader to the topic, it must include information on what the gist of the subject is, the methodology used (in the case of a science or social science study) and the conclusions drawn.
APA style requires the heading to be "Abstract" centered on the page (with the running head.) This should be page 2 of your document.
Do not indent the first line. Begin with a summary of the key points of your research. The abstract must cover all elements of the study: research topic, research questions, participants, methods, results, data analysis and conclusion. Here's the hard part. You must accomplish this with fewer than 250 words.
This means you must be exceedingly concise. The abstract does not include references. The key points are contained in a single sentence. The research questions may be limited to the main questions, without subquestions. The results will only be one or two sentences. You probably want to have about half the abstract focused on data analysis and conclusion.
At the end of the abstract, you can provide a list of keywords. These will be the words that will be used to find the document when it is listed in a database. This is somewhat anachronistic, and dates back to when things were filed in hard copy. Since today, search engines can see every word in a document and make intelligent decisions about what the keywords are, providing a separate list is not critical for researchers to find the document. However, the practice continues.
Indent the keyword line. The first word should be Keywords (in italics). It's followed by a colon and then a list of keywords, separated by commas.
Keywords: abstract, APA, essay writing, executive summary
The purpose of the abstract is to summarize everything of interest in the paper so someone researching similar topics will know whether or not to read your whole paper as a source of ideas for theirs. Details are not as important as the precision about what you have written.
In a business document, a similar opening page is called the Executive Summary. An executive summary should also be short, but there is no defined limit as in APA. I'd suggest that 300 words should be enough. It's important to define the topic being discussed, identify the important factors, and provide a brief summary of the important discussions and conclusions. In no case should the executive summary be longer than one page. The point is that someone should be able to read this brief page in a few minutes to understand the whole of what the document will lay out.
Image courtesy of athiwat at FreeDigitalPhotos.net