One of the distinctive features of the noun is its ability to own something, to possess something. We show this act of possession by adding a possessive ending, typically an “apostrophe ‑s” (Fred's report) to a singular noun and “just an apostrophe” to a plural noun (the persons' rights) (but children's toys).
Here's the rule for nouns: Form the possessive of all singular nouns, even those ending in ‑s by adding “apostrophe ‑s.” Many writers make the mistake of adding just an apostrophe to form the possessive of singular nouns ending in ‑s. This is rule 1, page 1 of Strunk & White's The Elements of Style. Thus, the following are correct:
the boss's priorities Congress's committees Bridget Jones's Diary
Theoretically, an inanimate object or abstract idea cannot possess anything, but writers routinely use possessive endings with inanimate objects, as in the rocket's red glare. These are technically called false possessives.
Some of the indefinite pronouns form their possessives with a possessive ending: another ('s), anybody ('s), anyone ('s), each one ('s), either ('s), everybody ('s), everyone ('s), neither ('s), no one ('s), nobody ('s), one ('s), other ('s), (others'), and somebody ('s.)
But other pronouns have special forms to show possession: my, mine, our, ours, his, her, hers, its, their, and theirs. Also, make sure you form the possessive of the neuter pronoun it like this: its. Many people incorrectly use it's and write things like We enjoyed it's plot. Wrong. We enjoyed its plot.