TL;DR:
socket.send(data, callback) is essentially equivalent to calling socket.emit('message', JSON.stringify(data), callback)
Without looking at the source code, I would assume that the send function is more efficient edit: for sending string messages, at least?
So yeah basically emit allows you to send objects, which is very handy.
Take this example with socket.emit:
sendMessage: function(type, message) {
socket.emit('message', {
type: type,
message: message
});
}
and for those keeping score at home, here is what it looks like using socket.send:
sendMessage: function(type, message) {
socket.send(JSON.stringify({
type: type,
message: message
}));
}