An apostrophe is a punctuation mark which is used to show that part of a word is missing, to show that something belongs to someone, or added after plurals of numbers, letters and abbreviations.
In the phrase "John's sentence", there is an apostrophe after John but before the "s" to show that the sentence belongs to John.

Punctuation marks (edit)

apostrophe ( ' ) ( ’ ) • brackets ( [] ) • braces ( {} ) • colon ( : ) • comma ( , ) • dashes ( – ) ( — ) • ellipsis ( ... ) • exclamation mark ( ! ) • hyphen ( ‐ ) ( - ) • parentheses ( () ) • period / full stop ( . ) • question mark ( ? ) • quotation marks ( ' ' ) ( " " ) • semicolon ( ; ) • slash ( / )

Other: ampersand ( & ) • asterisk ( * ) • at ( @ ) • backslash ( \ ) • bullet ( • ) ( · ) • degree ( ° ) • number sign / pound sign ( # ) • tilde ( ~ ) • underscore ( _ ) • vertical bar / pipe ( | ) •

Noun