s for plurals ("there are several contractors")
's for possessive, or to append 'is' or 'has' to a word ("a contractor's pair of boots", "the contractor's off to work" or "she's got lots of bananas")
' also for possessive but only if the possessing subject already ends in s (i.e. "several contractors' pairs of boots" or "those are Jess' boots")
The ONLY time this should be confusing, is when using its and it's. In this case, 's is ONLY for shortening of is ("it's cold, isn't it?"). The possessive form ("its boots are made for walking") has no apostrophe, as the word it is a pronoun and changes into its, in the same way that his/hers just changes for he/she.
Edit: Actually he/she/it don't translate perfectly to eachother, but you can see what I mean...
What is that over there? It's him/her/that
Who left the washing up? He/She/It did
Whose scarf is that? His/Hers/Its
But it's her scarf, not hers. Hrrmm.
Could go on forever. I sympathise with people who have trouble with all this, as a lot of it is just gut feeling for me nowadays. When I go back to Sweden, I have similar problems in Swedish (am bilingual) as I'm out of practice. A lot of it is just learnt through it /feeling/ right, and if you don't read or speak or write (in a situation where it needs to be correct) a language enough, you can lose that familiarity.