You know what an apostrophe is. It’s the little squiggle above the baseline in don’t and it’s that substitutes for a missing letter (o and i, respectively). Or it’s the little squiggle that denotes possession, because there is in fact a missing letter in those words: In Chaucer’s time, genitives (the linguistic term for what we casually call possessives) were formed by inserting an e before the s (the doges bone). We get apostrophe from Greek, with stops in Latin and French; the original form means “avert, turn away,” which is why we also use apostrophe for the rhetorical device of “turning away” to briefly address some person or thing—as one often does, say, on Twitter.
And speaking of Twitter, here’s what an apostrophe is not: It’s not a hyphen.