This punctuation mark has two main functions: First, it often shows possession: Dan Lindler's appointment. And second, it often marks the omission of letters in contractions and other words or numbers in years and decades: he'll, won't, finger lickin' good, the class of '68, the '90s.

Apostrophes never make a word plural, but they may be used to mark the plural of single letters and abbreviations with internal punctuation: Dot your i's. She got straights A's on her report card, M.A.'s Ph.D.'s. Don't use it in forming plurals of decades: the '70s, the 1980s, not '70's, the 1980's.