Use apostrophes to indicate possession. There are two basic methods that make use of an apostrophe in constructing the possessive. Most words use an apostrophe followed by an "s" at the end of the word, although many situations require simply an apostrophe.
Place an apostrophe before the "s" when you are indicating a singular possessive.
"Jacob's shoes are very cool." The shoes belong to Jacob (singular: one person).
"I found the dog's old bone buried in the backyard." The bone belongs to the dog (singular: a single dog).
Place an apostrophe after the "s" when you are dealing with a possessive plural case that has an "s" at the end (e.g., book to books, tree to trees). But if the word is plural without an "s" at the end, this rule does not apply; add an apostrophe and an "s" as if the word were singular.
"Look at all of the sailors' boats!" The boats belong to the sailors (plural: there is more than one sailor).
"The children's dresses were pink and frilly." The dresses belong to the children, but since the word children is already plural without having to add an "s" at the end, this is an exception.