Does anyone else cringe every time you read stuff like the title of this thread? In case you're wondering, it should read "Possessives and Apostrophes." If you're unsure about how this works and would like to write more correctly, please read on.
Here are rules for the use of apostrophes:
Use them only in possessives or contractions. NEVER use them in a plural that isn't a possessive like I did in the title of this thread.
In possessives, use the apostrophe between the word and the "s" only when it is singular. For instance, in the sentence, "My ski's binding broke," there is one ski, with one broken binding, therefore you put an apostrophe and then an "s." With more than one ski (plurals) put the apostrophe after the "s" like this: "My skis' bindings are constantly pre releasing." If a word already ends in an "s" you can either choose to just use an apostrophe after the word, or add an apostrophe and then an "s." Here are the two ways: "John Andrews' ski came off," or, "John Andrews's ski came off." They are both correct.
In contractions you use it in place of the missing letters. Examples: can not becomes can't, does not becomes doesn't, it is becomes it's.
The only exception to these rules are in the words its and it's. Since "it's" is a contraction, you don't use the apostrophe in the possessive "its." For instance: "My ski is lost. Its powder strap isn't showing up on the surface."
I know most people don't care, but this drives me nuts sometimes so I had to vent.