All about apostrophes

Apostrophes have three functions in a sentence:

• to form possessives of nouns
  

[see blog entry "Possessive nouns, or who owns what," July 22, 2009]

• to show where one or more letters or numerals have been left out

The most common use of the apostrophe to show omitted letters is in the contraction, when two words are combined and the second word loses one or more letters. Contractions are common in everyday speech and have become common in writing as well. Contractions are used frequently in less formal writing and less formal or not at all in formal writing.

can't [joining the words can and not, leaving out the letters n and o]
I've [joining the words I and have, leaving out the letters h and v]
didn't [joining the words did and not, leaving out the letter o]

The apostrophe is also used to show the omission of one or more letters at the beginning of a word or at the beginning of a combination of two words.

'twas [the letter i left out of the combination of the words it and was]
'cause [the letters b and e left out of the beginning of the word because]

The apostrophe is also used to show the omission of numerals, most often when referring to years.

the class of '09 [the numerals 2 and 0 are left out of the year 2009]

• to form the plural of a numeral, sign, letter of the alphabet, or word referred to as a word.

How many 0's are there in a million?
Indicate the most important points by marking them with *'s.
There are only two l's in the word follow.
You have too many too's in this paragraph.

In Brief: Use apostrophes to form noun possessives, to indicate where one or more letters or numerals have been left out, and to form plurals of numerals, signs, letters, and words referred to as words.