External JavaScript SDK is SDK for Mobile HTML5 applications and external browser-based apps. It can be used for client OAuth authorization for external applications, to invoke UI payment methods (just as JS SDK) for Mobile HTML5 applications, publish topics via publishing widget, send notifications or invite users to a game, and to invoke REST API methods

Android SDK is used for native Android applications. It allows to authorize users via client authorization, publish new posts, invite friends to a game, send notifications and invoke REST methods.

iOS SDK is used for native iOS applications. It allows to authorize users via client authorization, publish new posts, invite friends to a game, send notifications and invoke REST methods

Unity SDK allows to deeply integrate you Android/iOS app that uses Unity with OK