The short form of "you are". Used constantly in speech and in relaxed writing; in formal writing people often spell it out as "you are". It works for one person or several, since English "you" covers both. This is one of the most misspelled words in English — even native speakers write "your" by mistake, and you will see it wrong on signs and online every day. The test that always works: if you can replace it with "you are" and the sentence still makes sense, it takes the apostrophe. "You're late" = "You are late", so it needs the apostrophe. "Your bag" cannot become "you are bag", so it does not. In speech the two sound almost identical, which is exactly why the writing goes wrong. It appears in many fixed friendly phrases: "You're welcome", "You're kidding", "You're right".