Q. What is a DOI? Do I need to include it in my citation?

A Digital Object Identifier (DOI) is a unique number that can be assigned to online documents. You can think of them as HK identity card numbers for documents. They are designed to be permanent identifiers that will take you directly to the document no matter where it exists, even if its current URL changes.

Any type of document can have a DOI, but they are commonly found in journal articles. E-books can have DOIs too.

Whether or not you need to include the DOI in your citation depends on which style you are using:

APA style

The APA guidelines encourage the use of DOIs when they are available. Simply add it to the end of your citation. You should add the DOI in the manner prescribed by CrossRef, where the DOI itself is preceded by "http://dx.doi.org/" so that it resolves into a user-friendly URL, e.g. http://dx.doi.org/10.1080/00986280802374609

If your source doesn't include a DOI, don't worry about it as they are not a must-have.

The APA Style blog has covered the DOI in quite a lot of detail. Check out the following articles for more information:

• A DOI Primer 
  A more detailed introduction to DOIs
• A DOI and URL Flowchart
  An extremely handy flowchart that will help you determine when to include DOI, URL, or database information in your citation
• How to Use the New DOI Format in APA Style
  Covers recent changes in the use of the DOI

MLA Style

At the time of writing (December 2014), MLA style does not require the inclusion of the DOI.

Chicago Style

Again, DOIs are not required, but the Manual (section 14.184) does say that they are preferred to URLs. Include the DOI if you have it.