DOI = Document Object Identifier
A DOI identifies an individual article (like a serial number) such that a permanent URL can be created.
Most major publishers have started associating DOI's with articles and have started to show the DOI in abstracting and indexing databases (like Compendex) as well as on the article abstracts themselves.
Here's what one looks like:
The first part before the / is the publisher portion of the number. This one points to Elsevier. The second part points to the journal and article specifically.
if your citation includes a DOI, use it and omit the "retrieved from ..." statement.
Author’s Last Name, Initial(s). (Year). Article title. Journal Title (in italics), volume [also italicized], and issue number [in parentheses], page numbers.
Bordeck, N. (1997). Going against the grain: Disruptive classroom behavior. Behavioral & Educational Sciences, 19, 198-207.
Monthatipkul, S., Iovenitti, P., & Sbarski, I. (2012). Design of Facial Impact Protection Gear for
Cyclists. Journal of Transportation Technologies, 2 (3), 204-212. doi: 10.4236/jtts.2012.23022
(since there is a DOI, the URL is omitted)
Marcus, C. (2009). A 4-year, cluster-randomized, controlled childhood obesity prevention study: STOPP. International Journal of Obesity. 33 (4), 408-17. doi:10.1038/ijo.2009.38
(Citation includes DOI - so name of the database or URL is omitted)
Bindler, R. C., & Ball, J. W. (2007). The Bindler‐Ball healthcare model: A new paradigm
for health promotion. Pediatric Nursing, 33(2), 121-126. Retrieved from Proquest.
(Includes "retrieved from" and the name of the database. Note: this example does not have a DOI)