What are the correct uses of an apostrophe?

The apostrophe is a punctuation mark used in grammar. An apostrophe may indicated possession. For instance, if a cat has toys that are dirty, a person would say, "The cat's toys are dirty." Apostrophes are never used to indicate a plural form. For instance, the phrase "There are flowers in the garden" would never be "There are flower's in the garden". Apostrophes are also used in conjunctions. For example, the two Words "does not" becomes "doesn't". The Words "you are" becomes "you're".