An abstract is a concise (usually about 200-500 words) summary of the important points in a much larger document.  You may be required to write an abstract for a journal article or for a dissertation.

The length and content of your abstract will depend on the rules of your journal or the writing style that your field prefers.  The abstract should not necessarily be a miniature replica of the larger document; instead, give a brief explanation of how your research fits into the literature of the field, detail your findings, and emphasize their significance.