Pusher is a hosted API for quickly, easily and securely adding scalable realtime functionality to web and mobile apps.
WebSocket is an API built on top of TCP sockets and a protocol for bi-directional, full-duplex communication between client and server without the overhead of HTTP.
|Technology bidirectional more||
"The underlying websocket technology is bidirectional more info here but pusher s pub sub model is unidirectional"
from question "Am I using websockets correctly?"