Sometimes an author wishes to reference just an abstract, rather than a complete article. This is often the case for papers presented at conferences, where the abstracts for each paper, but not the papers themselves, are published in the Proceedings of the conference (a book summarizing the conference).
Sometimes the abstract is from a published paper, but the abstract is obtained from a secondary source, such as PsycINFO. Although abstracts may be cited as the information source for a paper, it is better to obtain and read the entire paper and cite it as the source of information.
Nevin, J. A., & Grace, R. C. (1998). Response strength, reinforcer omission, and resistance to extinction. Proceedings and abstracts of the annual meeting of the Eastern Psychological Association, 69, 56.
Preval, H., & Pakyurek, A. M. (1997). Psychotic episode associated with dexfenfluramine. American Journal of Psychiatry, 154, 1624-1625. Abstract obtained from PsycSCAN: Clinical Psychology, 1998, 19(2), Abstract No. 15.