Traditional Bitcoin mixers work by sending money to a third party who sends back different money. The problem with this is that sometimes they don't send back the money, and they can be anonymized. CoinJoin, on the other hand, is a way of mixing payments with a whole bunch of other people's payments. It creates lots of new UTXOs and scrambles the history of the coins, making them untraceable. The coordinator cannot steal the money, and it cannot be anonymized.