A hyphen is the sign (-) used to join words to indicate that they have a combined meaning or that they are linked in the grammar of a sentence (as in pick-me-up, rock-forming), to indicate the division of a word at the end of a line, or to indicate a missing or implied element (as in short- and long-term).
Hyphens help clarify meaning. For example a light purple bag is different from a light-purple bag. Without the hyphen you are saying that you have a bag that is both purple and light. With the hyphen you are saying that your bag is light-purple in color.
Or let's say you've got twenty two year olds. Without a hyphen, it is not clear if you mean that you run a day care and have twenty two-year-olds, or if you have twenty-two year olds applying for the job of nanny.
Hyphen errors simply come from putting then in the wrong spot or omitting them altogether. Here are a few quick tips:
1. For adjectives, the trick is to make sure that words you've got linked by the hyphen are being used as one word to modify the noun. When both words seperately modify the same noun, they are not hyphenated.
2. For nouns (and as a second check on all hyphens) a good dictionary is your best bet.
3. Do not use spaces around hyphens unless you have a list where one hyphenated item is being split: The pre- and post-game specials are a waste of time.
4. Hyphenate simple fractions: one-third or two-fifths.
5.Hyphenate compound numbers from 21 through 99: twenty-one, thirty, one thousand six hundred seventy-eight.
Source: Dictionary Version 2.1.2 (80.3) Copyright © 2005–2009 Apple Inc. and http://www.flickr.com/photos/hayzeleyes/4954905220/