Digital Object Identifers are a string of numbers and/or letters used to identify articles, books and more. An overview article about the DOI system will provide an explanation. If someone gives you a DOI then you can easily obtain the complete citation by typying the DOI string into the search box on www.doi.org.
Some citation manuals have you give the DOI in the citation. For example, the sixth edition of the APA manual uses DOI when citing online articles. You may be able to find the DOI in the first step. If the DOI is unavailable then use a url instead. You will need to follow step two to find the url of the periodical (magazine, journal, newspaper). The last resort would be to use the url of the library database which is step three.
Citations Manuals differ how to handle digital object identifiers. Below is an example from APA, 2010, sec. 6.31
Hopwood, C.J. & Morey, L.C. (2007). Emotional problems suppress disorder/performance in adult ADHD assessment. Journal of
Psychopathology and Behavior Assessment, 30, 204-210. doi: 10.1007/s10862-007-9067-8
If doi Not Available: Cite the URL for the journal homepage (even though journal articles may be available only by subscription). A quick Google or Ask search using the journal title will help you locate this link.
Antai-Otong, D. (2008). The art of prescribing: Pharmacological management of adult ADHD: Implications for psychiatric care.
Perspectives in Psychiatric Care, 44(3), 196-201. Retrieved from http://www.wiley.com/bw/journal.asp?ref=0031-5990&site=1