An abstract is a succinct summary of a longer piece of work, usually academic in nature, which is published in isolation from the main text and should therefore stand on its own and be understandable without reference to the longer piece. It should report the latter's essential facts, and should not exaggerate or contain material that is not there.
Its purpose is to act as a reference tool (for example in a library abstracting service), enabling the reader to decide whether or not to read the full text.