Your or you're: what is the actual rule?

You're is short for you are, and the apostrophe marks the missing letter. Your indicates possession, something belongs to you. The test is mechanical and never fails: read the sentence with you are substituted in. You're going to love this expands to you are going to love this, which works, so the contraction is correct. Your car is blocking me expands to you are car is blocking me, which is nonsense, so the possessive is correct.

The same test resolves the entire family of homophone traps that generate the most online arguments. It's is it is or it has; its is possessive, and unlike most possessives it takes no apostrophe, which is precisely why the error is so common. They're is they are, their is possessive, there is a place or the dummy subject in there is. Who's is who is or who has; whose is possessive. In every pair, expand the contraction: if the expansion reads correctly, the apostrophe version is right. Autocorrect will not catch these, since both spellings are real words, which is why they survive spellcheck and end up in published work and, more consequentially, on résumés and cover letters.