The Apostrophe: A Quick and Dirty Guide to Usage

ARGHHHHH!!! (Photo credit: Wikipedia)

Why? Because it's one of my pet peeves, that's why. I'm that guy who sneaks up to restaurant menu whiteboards and rubs his thumb over apostrophes that don't belong. The fist you see shaking through a car window when some professional sign painter has put an apostrophe where it doesn't belong? Almost certainly mine. So ...

• The apostrophe IS NOT used to make singular nouns into plural nouns. "I have two ponies" is correct. "I have two pony's" is incorrect.
• The apostrophe IS used to create contractions. "I'm" is a contraction of "I am." "Don't" is a contraction of "do not." And so on and so forth.
• The apostrophe IS used between the end of a singular noun and the letter "s" to indicate possession, with three exceptions. "Bob's dog" and "Betty's car" are correct. "Bobs dog" and "Bettys car" are incorrect.
• The apostrophe IS used after the "s" ending a plural noun to indicate possession. If three ponies have bridles, they are "the ponies' bridles." 
• The three exceptions to the usage above are ancient proper names ending with "s"  ("Jesus' name," "Achilles' heel"), possessive pronouns ("hers" and "its," not "her's" and "it's" -- that last one is properly used as a contraction for "it is") and stilted uses of the word "sake" after words ending in "s" (e.g. "for righteousness' sake" ... but please, please, please, just go with "for the sake of righteousness" instead).
• Note the word "ancient" above. For modern proper names that end in "s," use an apostrophe and an additional "s" -- use "Charles's head" and "Congress's prerogative," not "Charles' head" and "Congress' prerogative."

There you have it. If you don't want to accept the rules above on my authority, feel free to confirm with Strunk and White.