Most smartwatches are essentially smartphone accessories because they depend on a connection to your smartphone via bluetooth or a Wifi network. The exceptions are the smartwatches with built-in cellular data connections.  

Paired with your smartphone, smartwatches can notify you—via vibration, sound, or both—of incoming calls, texts, instant messages, social-media posts, and other app alerts. Many watches also display messages on their screens. 

Many smartwatches let you respond to incoming e-mail, texts, and messages. Some offer pre-populated replies, such as "Sorry, I'm busy right now" or "I'll call you back later."
Some let you accept, dismiss, and even initiate phone calls. Some smartwatches let you conduct phone calls—mostly on your paired phone, but a few directly via the watch itself.

 

Many smartwatches measure your steps and movements. Some have built-in heart-rate monitors, barometers, and GPS, and many work with fitness apps that keep records of your activity and help you track your calorie intake and calories burned. Some claim to measure the length of time you sleep at night and even how restful your sleep was.

The Apple Watch and smartwatches that run on Android Wear let you use spoken commands to do internet searches, send texts, access watch apps, and more. Some other models also include voice-command capabilities.

Smartwatches, like other smart devices, can run apps, via your phone or right on the watch. These include apps for health and fitness, navigation, weather, social media services, and much more. And developers are coming up with new apps that can increase the functionality of these devices. The range of apps varies, depending on the operating system.