Overview

Apps developed in Android Wear, Tizen, and watchOS can be either standalone or companion type.  Standalone apps run on the device itself while companion apps primarily run on the phone and the watch displays the UI.  Those that require data can fetch it over Bluetooth, Wi-Fi connection, or cellular in the case of the Samsung Gear S/S2 and LG Urbane 2nd Edition LTE.

Aside from availability of third party applications, the main difference between platforms is in their abilities to function without a nearby phone.  Tizen wearables with cellular radios such as the Samsung Gear S2 3G are fully functional even with the phone powered off.  Contrast that with Android Wear and watchOS devices, which still require the phone to be connected to a network or nearby for full functionality.  Features such as Google Now and Siri are unavailable if the phone is disconnected from the network.

Platforms

Android Wear

Android Wear applications are packaged within a companion application.  When users download the companion application from an Android store, the wearable application is automatically pushed onto the Android Wear device.

Tizen

Tizen wearable devices are paired with a host device, such as a smartphone, which they may depend on for wider connectivity and various services.  The host runs the Gear Manager application, which communicates with the Wearable Manager Service on the wearable device.  Both of these are used for managing applications on the wearable device.

Standalone applications are Gear applications based on Tizen.  They do not need a general host application, and can operate independently.

Companion applications are comprised of two parts: a Gear application and a corresponding application running on the host, usually operating as a consumer/provider pair.  Host applications are developed using Android APIs, while Gear applications are developed using Tizen.  You can use companion type applications when your Gear application needs data communication or interaction between a host-side application and a Gear application.

Host-side applications need to be uploaded to Android stores, such as Google Play Store, and Gear applications need to be uploaded to Samsung Gear Apps.

watchOS

Watch OS applications consist of two separate bundles: a Watch app and a WatchKit extension.  The Watch app bundle contains the UI for the application.  The WatchKit extension bundle contains the code for the application.  Apps developed in watchOS can run locally on the device, but they must be bundled with a companion iOS application.

Table: Feature list between Tizen and watchOS

Feature	Platform
	Android Wear	Tizen	watchOS
Standalone Apps	Yes	Yes	No
Companion Apps	Yes	Yes	Yes
App Installation Method	Phone app	Store or Phone app	Phone app
Android Phone Compatible	Yes (Android 4.3+)	Yes (Gear S2 with Android 4.4+ device)	No
iPhone Compatible	Maybe (depends on model)	In development (Gear S2)	Yes
Development Language	Java or C/C++	HTML5 and Javascript (Web app) C or C++ (Native app)	Swift or Objective-C
Host Platform for Companion Apps	Java	Java for Android (with accessory SDK)	Swift or Objective-C
Recommended IDE	Android Studio	Tizen SDK for wearables	Xcode (Mac-only)

References

http://developer.samsung.com/gear/gear-develop-overview

https://developer.apple.com/library/ios/documentation/General/Conceptual/WatchKitProgrammingGuide/DesigningaWatchKitApp.html

http://officialandroid.blogspot.com/2015/11/cellular-support-comes-to-android-wear.html

http://developer.android.com/training/wearables/apps/creating.html