1. Singular nouns that end in "s"
It tends to confuse people when a singular noun ends in "s" but needs to show possession. Often, the temptation is to simply stick a dangling apostrophe at the end of the word. The CORRECT thing to do, however, is to add the apostrophe-s to the end of the word, just as you would if the word did NOT end in an S:
- Rufus's backpack NOT Rufus' backpack
- the boss's wife NOT the boss' wife
There are 2 notable exceptions: It is generally accepted that JESUS' and MOSES' are correct possessive forms, simply because Jesus's and Moses's sound like you've got a speech impediment.
2. Plural possessives
A plural possessive is a word that shows ownership by more than one person. Here are the rules:
If the plural noun ends in an "s", simply add the apostrophe at the end.
- girls' (belonging to more than 1 girl)
- teachers' (belonging to more than 1 teacher)
- friends' (belonging to more than 1 friend)
- foxes' (belonging to more than 1 fox)
If the plural noun has its own, special form, add an apostrophe-s the way you would with a single noun.
That about covers it. I highly recommend Lynne Truss's Eats, Shoots, and Leaves for an entertaining and definitive source of apostrophe (and comma) wisdom.