The apostrophe is used as follows:

1. to indicate the omission of letters or numerals:
   
   
   can't – cannot
   
   doesn't – does not
   
   
   Be careful to distinguish between it's as a contraction of it is, and its, the possessive pronoun. The first takes the apostrophe; the second does not.
   Note that the apostrophe is not used with dates such as the 1980s or plural abbreviations such as PGEs.
   
   
   
2. to form the possessive of nouns not ending in an s or z sound:
   
   
   formation's boundaries
   cliff's edge
   river's course
   
   
   If the noun ends in an s or z sound, the apostrophe alone is used for the possessive:
   
   
   Davey Jones' Locker
   fines' characteristics
   provinces' resources
   forests' trees
   
   
   
   The apostrophe is often omitted in instances where the word is not used in a truly possessive sense:
   
   
   Prospectors and Developers Association
   several minutes delay
   seven days leave