A fitness tracker is essentially a bracelet with sensors that send data to your phone to provide an expanded picture of the data it collects, and usually requires a phone for GPS and music. A smartwatch is essentially a fitness tracker plus your phone, but on your wrist. It has GPS built-in (sometimes using cellular data, which usually just acts as an extra line on your smartphone carrier bill) and can download apps. With something like an Apple Watch, you can send voice messages or take calls, have a conversation via iMessage, and get all your social media notifications to your wrist.