This article is from the alt.usage.english FAQ, by Mark Israel email@example.com with numerous contributions by others.
The apostrophe in these cases normally has no effect on
pronunciation. Thus dogs, dog's, and dogs' all sound the same. The
exception is where the apostrophe separates two "s"s, and then it is
pronounced as an unstressed schwa. Thus class's, classes, and
classes' are all pronounced as /klA:s@z/.
For nouns where there is some difference of opinion over whether
the possessive suffix should be -'s or a bare apostrophe (that is,
those nouns where a final unstressed syllable ends with an [s] or
[z] sound) some native speakers use a lengthened final consonant
intermediate between /z/ and /z@z/. This is, however, a fine and
almost inaudible distinction.
One occasionally hears that "John's dog" is an abbreviation for
"John his dog". It is more likely that the derivation went in the
opposite direction, i.e.:
Johnes hund => John's hound => Johnny's dog => John 'is dog
with the "John his dog" form coming into use only briefly before
disappearing from modern English.
Using an apostrophe in a plural which is not a possessive form is
almost never recommended by prescriptivists. The only situation
where it is recommended is where visual confusion would otherwise
result, as for example in the sentence "Mind your p's and q's". In
forms like "the 1980s" or "two CPUs", apostrophes are not
It is correct to use an apostrophe to indicate missing letters,
in contractions like "aren't", "isn't", "it's" (= it is or it has).
Be careful in these cases to put the apostrophe in the correct
place. The apostrophe replaces the missing letter(s); it does not
replace the space between words.