First let's all join in a hearty curse of the grammarians who inserted the wretched apostrophe into possessives in the first place. It was all a mistake. Our ancestors used to write "Johns hat" meaning "the hat of John" without the slightest ambiguity. However, some time in the Renaissance certain scholars decided that the simple S of possession must have been formed out of a contraction of the more "proper" "John his hat." Since in English we mark contractions with an apostrophe, they did so, and we were stuck with the stupid "John's hat." Their error can be a handy reminder though: if you're not sure whether a noun ending in S should be followed by an apostrophe, ask yourself whether you could plausibly substitute "his" or "her" for the S.
The exception to this pattern is personal pronouns indicating possession like "his," "hers," and "its."
Get this straight once and for all: when the S is added to a word simply to make it a plural, no apostrophe is used (except in expressions where letters or numerals are treated like words, like "mind your P's and Q's" and "learn your ABC's").
Apostrophes are also used to indicate omitted letters in real contractions: "do not" becomes "don't."
Why can't we all agree to do away with the wretched apostrophe? Because its two uses -- contraction and possession -- have people so thoroughly confused that they are always putting in apostrophes where they don't belong, in simple plurals ("cucumber's for sale") and family names when they are referred to collectively ("the Smith's").
The practice of putting improper apostrophes in family names on signs in front yards is an endless source of confusion. "The Brown’s" is just plain wrong. (If you wanted to suggest "the residence of the Browns" you would have to write "Browns'," with the apostrophe after the S, which is there to indicate a plural number, not as an indication of possession.) If you simply want to indicate that a family named Brown lives here, the sign out front should read simply "The Browns." When a name ends in an S you need to add an ES to make it plural: "the Adamses."
No apostrophes for simple plural names or names ending in S, OK? I get irritated when people address me as "Mr. Brian's." What about when plural names are used to indicate possession? "The Browns' cat" is standard (the second S is understood), though some prefer "the Browns's cat." The pattern is the same with names ending in S: "the Adamses' cat" or -- theoretically -- "the Adamses's cat," though that would be mighty awkward.
Apostrophes are also misplaced in common plural nouns on signs: "Restrooms are for customer's use only." Who is this privileged customer to deserve a private bathroom? The sign should read "for customers' use."
It is not uncommon to see the S wrongly apostrophized even in verbs, as in the mistaken "He complain's a lot."