From the calculator watch
Digital calculator watches were a hit in the 1980s. In 2012 Pebble raised more than 10 million dollars from fans and showed that people wanted a phone on their wrist. Apple, Samsung and Google followed with their own watches.
Health
Watches measure heart rate with an optical sensor; some can take a simple ECG, measure blood oxygen or detect a fall and call for help. There are known cases where a watch warned of an irregular heartbeat. They are not full medical devices, though, and do not replace a doctor.
Battery
Battery life is the main difference: an Apple Watch lasts about a day or two, Garmin sports watches for weeks.