An abstract is a summary of the larger work that reports on the main facts. From the abstract, a reader is able to discern the content of the rest of the document. It also helps in indexing work.
An abstract, which is usually academic in nature, is complete on its own. A reader can understand the work without referring to the larger document. This helps him decide whether or not the larger document is of value to him.
An abstract is important in indexing; therefore, the writer should include keywords to make searches easier. Typically, it contains the purpose of the study, the methodology, the findings and the conclusion.