JavaScript SDK
JavaScript SDK is SDK for built-in applications that are iframes, the content of which is located on the developer’s server.

External JavaScript SDK
External JavaScript SDK is SDK for MOBILE_HTML 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_HTML applications, publish topics to user’s wall via publishing widget, send notifications or invite users to a game, and to invoke REST API methods.