Apostrophes indicate possession, represent letter omission, and indicate certain plurals. These are the universally accepted uses of the apostrophe:
Apostrophes come before an S after a noun or proper noun to indicate that it is possessive.
Note: "It's" is a contraction of "it is"; "its" is a possessive pronoun much like "his" or "her."
Apostrophes follow a plural noun or plural proper noun that ends in S to indicate that it is possessive.
Note: The Merriam-Webster Guide to Punctuation & Style extends this rule to plurals ending in Z.
If two or more nouns or proper nouns are expressing possession of the same thing, the rules above are applied to the final possessor.
If two or more nouns or proper nouns are expressing possession of separate things of the same name, the first two rules above are applied to all possessors.
When two words are combined to form a contraction, an apostrophe replaces the omitted letter or letters.
Note: "It's" is a contraction of "it is"; "its" is a possessive pronoun much like "his" or "her."
An apostrophe comes before an S after a letter to show that the letter is plural.