An apostrophe is used:
a. To show possession
Following a noun: an apostrophe plus an s.
e.g. The book’s cover.
But—following a noun that ends in s: an apostrophe only.
e.g. The puppies’ tails.
Exception—personal names that end in s add an apostrophe and an s.
(But—Japan Harvest adds only an apostrophe to the names Jesus and Moses.)
e.g. Jones’s; Keats’s; Jesus’; Moses’
Note: Possessive pronouns—including “its”—already show belonging, so do not need an apostrophe. (“It’s”—it is—indicates omission.)
b. To show omission
e.g. the ’70s, don’t, can’t, I’ll, it’s.
c. To form a plural with lowercase letters (but not numerals or uppercase letters)
e.g. x’s and y’s, but 3s and PhDs.
d. (Rarely) In a proper geographic name
e.g. Pikes Peak, but Martha’s Vineyard.
With only a few exceptions in the US and Australia, “apostrophes suggesting possession or association are not to be used within the body of a proper geographic name”¹. Perhaps more commonly used in Canada and Britain, the apostrophe usage debate continues to rage. Check an official map or article for proper spelling.