In fact, you've already seen that yourself in using online data bases such as ERIC and Periodical Abstracts. But abstracts also may accompany the document in final form. In this sense, they are kind of like a prospectus, but they don't have all a lot of the apparatus that a prospectus has. The script in discussion sections of theoretical papers is especially important for an abstract to replicate. Normally abstracts do not have quoted materials, and they need no documentation.
Abstracts are normally objective rather than evaluative.
Abstracts may consist of more than a single paragraph. Like many other genres, they ought to hold together from sentence to sentence through cohesive markers. The process for writing an abstract differs, depending on whether it's a preliminary one or one that draws from an already-existing document.