What does APA stand for?

Writing a research paper, are you? In a nutshell, APA stands for American Psychological Association. These good folks put together a massive reference book containing hundreds of guidelines on how to format references, statistics, tables, punctuation, and grammar. It also contains useful writing tips and formatting instructions. Most research papers and thesis papers are expected to follow APA style.
If you've been asked to do so in a paper, I'll bet you just thought your teacher was being nitpicky. But every publication needs to follow rules for clarity. For example, a writer could choose among many different ways to cite a date (October 31, 2005; Oct. 31, 2005; 10/31/05; and 2005-10-31 among them). Without a style guide, writers might cite the same date differently within the same publication. Although the date hasn't changed (in this case, October 31, 2005 follows APA style), such variations could be very distracting to the reader. More complex material, like tables or lists of statistics, requires even more need for consistent style.