Just because you are viewing the item online, doesn't mean it's a web site.
Online or electronic books are published once and are revised on a set schedule. Online books usually have a print equivalent that contains the exact information, but in a print form. Usually, these require payment to access. USC Libraries license many electronic books for use by USC affiliates, for example: students, staff and faculty.
Electronic journals are also published on a set schedule (usually monthly or quarterly). These also have print equivalents and require payment to access. USC Libraries license many electronic journals for use by USC affiliates.
Web sites are published once, then updated as information is developed or discovered. Web sites do not have a print equivalent. USC Libraries do license some web sites for use by USC affiliates, but the majority of web sites are free to all.
Refer to Citing Medicine for more examples and guidance regarding NLM style.
Although online materials often differ than their physical counterparts, the method for citing them is not that different. Remember, it is not sufficient to simply list the URL or DOI for an online material.
Journal article on the Internet
Lie D, Carter-Pokras O, Braun B, Coleman C. What do health literacy and cultural competence have in common? Calling for a collaborative health professional pedagogy. J Health Commun [Internet]. 2012 Oct 3 [cited 2012 Dec 3]; 17 Suppl 3: 13-22. Available from: http://www.tandfonline.com/doi/full/10.1080/10810730.2012.712625
Book on the Internet
Mahmood T, Arulkumaran S. Obesity: a ticking time bomb for reproductive health [Internet]. Waltham (MA): Elsevier; c2013 [cited 2012 Nov 21]. 658 p. Available from: http://www.sciencedirect.com/science/book/9780124160453
Chapter within a book on the Internet
Heimburger DC. Chapter 75, Malnutrition and nutritional assessment. In: Long DL, Fauci AS, Kasper DL, Hauser SL, Jameson L, Loscalzo J, editors. Harrison's online [Internet]. 18th ed. Columbus (OH): McGraw-Hill Companies; c2012 [cited 2012 Nov 20]. [about 9 screens]. Available from: http://www.accessmedicine.com/content.aspx?aid=9099972
Entire database on the Internet
MeSH Database [Internet]. Bethesda (MD): National Library of Medicine (US). 2003 Apr - [cited 2012 Jul 8]. Available from: http://www.ncbi.nlm.nih.gov/mesh
Part of a database on the Internet
Becker MA. UpToDate [Internet]. Waltham (MA): 2012 Feb 27. Prevention of recurrent gout; [cited 2012 Dec 5]; [about 16 screens]. Available from: http://www.uptodate.com/contents/prevention-of-recurrent-gout
Web site
Hooper JF. Psychiatry and the Law: Forensic Psychiatric Resource Page [Internet]. Tuscaloosa (AL): University of Alabama, Department of Psychiatry and Neurology; 1999 Jan 1 [updated 2006 Jul 8; cited 2007 Feb 23]. Available from: http://bama.ua.edu/~jhooper/
Blog
Neinstein A. Diabetes Technology (and other musings on digital health) [Internet]. San Francisco: Aaron Neinstein. 2012 Feb - [cited 2012 Sep 16]. Available from: http://diabetestechnology.wordpress.com/
Electronic mail
Backus, Joyce. Physician Internet search behavior: detailed study [Internet]. Message to: Karen Patrias. 2007 Mar 27 [cited 2007 Mar 28]. [2 paragraphs].