//On server side or server side
socket.send(someMessage);

//On another opposite side
socket.on("message", function(data){console.log(data);})

//On server side or client side
socket.send = socket.emit("message", someMessage);

//On opposite side
socket.on("message", function(data){console.log(data);})

//register a custom events on socket using socket.on function
socket.on("clientSentAData", function(data){console.log(data);})

//Client knows that there is a kind of evnets known as "clientSentAData" in the server side
//So client emit an data using "clientSentAData" event.
socket.emit("clientSentAData", data);