Apostrophes

Basically:

1. Possession, e.g.:
The carpenter's tools (one set of tools belonging to one carpenter)
The carpenters' tools (several carpenters; several sets of tools)

But NOT in possessive PRONOUNS, e.g.:
its, his, hers, theirs, ours


2. Omission of one or more letter in abbreviations, e.g.:
We're (we are)
They're (they are)
They've (they have)
It's (it is)