• Who:  The author, authors, or organizations responsible for the piece
• What:  Title of the piece
• When:  When the piece was published
• Where:  Where the piece was published or presented

If you're in doubt, the APA recommends keeping in mind the question of how you will get readers to the source.   -- Asking the Right Question: How Can the Reader Find the Source? on the APA Style Blog.

• Author(s)
• Year
• Article title
• Journal title
• Volume / issue
• Pages
• DOI (if available)

Example:

Grundy, J., & Hosking, J. (2002). Developing adaptable user interfaces for component-based systems. Interacting with Computers, 14(3), 175-194. doi: 10.1016/s0953-5438(01)00049-2

• Author(s)
• Year
• Book title
• Publisher
• Publisher location

Example:

Stephanidis, C. (2001). User interfaces for all : concepts, methods, and tools. Mahwah, NJ: Lawrence Erlbaum Associates Inc.
 

• Author(s)
• Year
• Title of the chapter
• Editor(s) of the book
• Title of the book
• Publisher
• Publisher location
• Pages   

Examples:

Akoumianakis, D., Grammenos, D., & Stephanidis, C. (2001). User interface adaptation: Evaluation perspectives. In C. Stephanidis (Ed.), User interfaces for all: Concepts, methods, and tools (pp. 339-352). Mahwah, NJ, US: Lawrence Erlbaum Associates Publishers.

• Author(s)
• Year
• Presentation Title
• Conference title
• Conference location   

Example:

Stuerzlinger, W., Chapuis, O., Phillips, D., & Roussel, N. (2006). User interface facades: towards fully adaptable user interfaces. Paper presented at the Proceedings of the 19th annual ACM symposium on User interface software and technology, Montreux, Switzerland.
 

• Author
• Date (use "n.d." if there is no date given)
• Title of document [format]*
• Source (the URL)

Often one of these pieces of information is missing.  Take a look at this handy chart from the APA (pdf) for help in figuring out what to do in those situations.

Example

Lee, C. (November 18, 2010).  How to cite something you found on a website in APA style [blog post].  Retrieved from http://blog.apastyle.org/apastyle/2010/11/how-to-cite-something-you-found-on-a-website-in-apa-style.html

* Note: you only need to give the format if it's something out of the ordinary.  You don't need to add it for a regular webpage or article.