Like the comma, the apostrophe wields amazing power. The talented little dot-and-tail combination (though written at the top of the line, not at the bottom like the comma) can change pronouns to verbs, tell you who owns what, replace a small handful of letters, and make plurals. It comes from the Greek words meaning to turn from or omission.
After commas, apostrophes seem to be the most misused punctuation mark; grammar vigilantes have their work cut out for them.
Earlier, we wrote about how apostrophes are used. Check out that post here.