One of the most distinct elements of the new Apple Watch is a set of four rings built into its backside, but they're hardly there for style: inside those four rings are sensors that the Apple Watch uses to measure its wearer's pulse. The sensors include infrared and visible-light LEDS in addition to photosensors, which all work together to detect heart rate. Using that information, Apple says that it can begin to put together a comprehensive look at a person's daily activity.
Though Apple is putting a focus on these four rear heart-rate sensors, there's a bit more than that at play. There's an accelerometer inside the watch that's likely used for tracking movement. And since the watch is connected to an iPhone, it also uses the phone's Wi-Fi and GPS data to see where you're going. There's a whole suit of health features built in as well, giving wearer's fitness goals and telling them how many calories they've burned.
The sensors won't just be used for your health, though: Apple is also having a bit of fun with the heart-rate monitor. The watch allows its wearer to record their heartbeat, and then send it to a friend, who will feel it played out for them in vibrations. It's obviously not a practical feature, but perhaps Apple sees it being used as a romantic gesture.