feat: add device docs

docs_src/microservices/device/details/DeviceDefinitions.md

@@ -3,5 +3,16 @@ title: Device Service - Device Definitions
 ---
 
 # Device Service - Device Definitions
+## Concept
+![image](Device.png)
 
-coming soon
+In EdgeX parlance, "device" is used to refer to a sensor, actuator, or IoT "thing". A sensor generally collects information from the physical world - like a temperature or vibration sensor. Actuators are machines that can be told to do something. Actuators move or otherwise control a mechanism or system - like a value on a pump. While there may be some technical differences, for the purposes of EdgeX documentation, device will refer to a sensor, actuator or "thing".
+
+Data about actual devices is another type of information and each device must have a unique name associated to it. The [core metadata micro service](../../core/metadata/Purpose.md) stores information about a device. This information is used by other services (Device, Command, etc) to communicate with the devices. Each device is also associated to a device profile. This association enables metadata to apply knowledge provided by the device profile to each device. For example, a thermostat profile would say that it reports temperature values in Celsius. Associating a particular thermostat (the thermostat in the lobby for example) to the thermostat profile allows metadata to know that the lobby thermostat reports temperature value in Celsius.
+
+## Usage
+### How to start
+You can [define your device](../../../walk-through/Ch-WalkthroughDeviceProfile.md) by creating device profile first and add your device based on the profile.
+
+### Provisioning
+Some device service have the ability to discover new devices, see [Provision a device](../../../walk-through/Ch-WalkthroughProvision.md) and [Device Discovery](../../../microservices/device/details/DeviceDiscovery.md) for more detailed information.