It is an original work, not an excerpted passage. An abstract must be fully self-contained and make sense by itself, without further reference to outside sources or to the actual paper. It highlights key content areas, your research purpose, the relevance or importance of your work, and the main outcomes.
It is a well-developed single paragraph of approximately 250 words in length, which is indented and single spaced. The function of the abstract is to outline briefly all parts of the paper.