An abstract is a summary of the work reported in a paper. It allows readers to quickly identify articles of interest to them, without having to read the whole document. Together with the title, the abstract is also used by databases to allow articles to be searched easily.
Abstracts should almost always be written last. It should briefly state the overall thesis and scope of the paper, describe the methodology employed, summarize the results, and reiterate the main findings. Most journals specify a word limit, which is less than 250 words.
Often when writing professionally, you will have to supply your own abstract. Even if you are writing a paper that will never be published, writing an abstract can help you solidify and summarize the ideas described in your paper.
When writing your abstract, never include information or conclusions that are not given in the paper. The abstract should be understandable by itself, as it will be published separately in abstracting services. The abstract should not contain abbreviations and references to literature and to figures and tables.
It may also be helpful to add a keyword list at the end of your abstract, which will be used by indexing and abstracting services. Consider adding keywords not already in the title or the abstract, so that readers find it easier to locate your paper.