apostrophe

An apostrophe shows some kind of possession (belonging to), eg "Lucy's cat", or it shows that a letter has been missed out from a word, eg "I'm for I am".  (Sometimes more than one letter is missed out, as in "they'd" for "they had.")

Try not to use an apostrophe to indicate a plural: "There were eleven player's in the team." This is not correct, although you will see a lot of it (especially in shops where it has become known as the "grocer's apostrophe").

To show possession in a word or name which ends in "s" you would normally add an apostrophe plus a second "s" (James's).  Plural nouns which don't end in "s" are given an apostrophe and an "s" ( old folk's home, children's toy, people's republic). 

Apostrophes used when letters are missed out are more usual in informal writing - they can change the tone of your writing.  Do not use this kind of apostrophe in an job application, or a UCAS application.