Publishing institutions -- including the Wall Street Journal, Random House, the U. S. Government Printing Office, my little Silver Sands Books and even this blog -- follow certain styles.
The styles dictate such things as spelling, capitalization, abbreviation and hyphenation, and are promulgated in The New York Times Manual of Style and Usage, The Associated Press Stylebook and The Chicago Manual of Style (from the University of Chicago Press).
The AP book is a reference book you can actually read for fun.
Sometimes the style books agree with each other. Sometimes they don’t. For example, "Chicago" (which was first published in 1891) favors the serial comma, but the AP and the Times books oppose it. Their attitude may be based on the need to save space in crowded newspapers. "Chicago" style is more often used by book publishers.
The language authorities often disagree. The Chicago Manual of Style tells us
that french fries and swiss cheese need no uppercase letters. The AP book says
we should capitalize Swiss cheese.
The AP book was first produced as a 60-page booklet in 1953. Over the years, this “Bible of the Newspaper Industry” grew considerably in both size and scope. The 2007 version that I use is still a rulebook, but it’s also a dictionary, an encyclopedia and a textbook. The AP updates annually. I'm still using the 2010 edition. The Chicago and Times books update less frequently. I have the newest Times book, published in 2009. My 15th edition of the Chicago book was published in 2003. The 16th edition came out in 2010.
In your own writing, you can pick which book you want to follow, or pick and choose bits and pieces from each. You can even disagree with all three and make up your own rules -- if you have a good reason to do so and don’t do anything too stupid. The best solution is to pick a style and be consistent in each book, blog or article you write. Don't have “3 a.m.” on one page and “5PM” 100 pages later.
The Oxford University Press and the Chicago Manual of Style insist that an em dash should be attached to the letters before and after it, like—this, with almost no visible space. On the other side, the New York Times likes to put a space before and after each em dash. I’ve gone back and forth on this issue, with different styles in different books. As long as I publish my own books, I control my em dashes. You control yours.
While a self-publisher can choose (or create) her or his style, if you get a contract from another publisher -- or even if you freelance for a magazine, newspaper or website -- you may encounter "house style." Random House may have different preferences than Simon & Schuster, and the New York Times may disagree with Esquire. Online, Slate, Salon and Huffington Post may have different standards -- and standards evolve over time.
Just as language does not stand still, neither do the official styles.
The AP recently switched from "web site" to "website" and endorsed "email" over "e-mail," "handheld" over "hand-held" and "cellphone" over "cell phone." I made those changes years ago. I may be a pioneer in style. Yesterday I wore slightly mismatched Crocs (indoors, only). Does that count as "style?" Is it any worse than jeans with holes in them?
Unlike "email," for now, the AP will continue to hyphenate other “e” terms, such as "e-book" and "e-commerce." Little is logical in the English language.