An apostrophe is normally used with the letter s to show ownership or possession.
With most singular nouns, simply add an apostrophe plus the letter s to do this.
An apostrophe plus s is never added to make a noun plural--even a proper noun.
Incorrect: This is Joans jacket.
(Possessive form needs the apostrophe)
Correct: This is Joan's jacket.
Incorrect: He ate four hot dog's at the picnic.
(Not possessive; use no apostrophe to make a noun plural.)
Correct: He ate four hot dogs at the picnic.
Incorrect: We saw the Smith's at the picnic.
(Not possessive; use no apostrophe to make a name plural.)
Correct: We saw the Smiths at the picnic.
If the singular noun ends with an s, add apostrophe s if the extra syllable is pronounced. If the extra syllable is not pronounced (or if it otherwise looks confusing to add apostrophe s), simply add an apostrophe.
Examples: the dress's hem
(Added syllable is pronounced.)
Lloyd Bridges' son
(Added syllable is not pronounced.)
Some authorities always add an apostrophe only to any word ending with s, regardless of its pronunciation. This is acceptable. Whichever standard you follow, be consistent.
Example: the dress' hem
(Word ends in s, pronunciation does not matter.)
See also Plural Possessives, Apostrophes with Possessives of More than One Owner, Apostrophes with Italicized and Underlined Items, and Apostrophes with Pronouns.