Before we begin, I must stress that you should never use an apostrophe to form an ordinary plural. Don't abuse the apostrophe.
This said, despite what you may hear from others, there are exceptions to the rule that plurals should not be formed with apostrophes. Specifically, you may use an apostrophe to form the plural of an abbreviation or a symbol. Thus,
"There were more Ph.D.'s at the reception than you could shake a stick at."
"Ice skater Valentina Semyovets received four 6.0's and six 5.9's for her astounding performance in the women's short program."
"Bullets over Biloxi is a riveting story of a gangster hideaway during the Roaring 20's."
"Mind your P's and Q's at the board meeting, do you hear me?"
are all perfectly good English usage, as is taught in high schools and colleges1 across the United States.
Over the past 20 years or so, usage in the United States has begun to move towards omitting the apostrophe when forming the plurals of abbreviations and symbols. The trend is perfectly alright; the extra punctuation is not needed. However, to characterize its omission as 'correct' is a misrepresentation.
This difference does not result from 'ignorance'. It does not even result from 'confusion'. This is a cultural difference, and you would be well-advised to treat it as such.
Another cultural difference involves forming possessives from words which
end in s. Some would assert that the 'correct' way to form such a possessive is to append an apostrophe but nothing else. This is decidedly not the case. The premier guide to English usage, The Elements of Style by William Strunk, Jr., and E. B. White, begins:2
I. Elementary Rules of Usage
Form the possessive singular of nouns by adding 's.
Follow this rule whatever the consonant. Thus write,
the witch's malice
1For an example, see http://owl.english.purdue.edu/handouts/grammar/g_apost.html
2Strunk, William; White, E. B. The Elements of Style, Third Edition, © 1979 Macmillan Publishing Co, Inc., New York.