Ways to connect client-side to server-side

When connecting the client to the server (the server written in node or one of its frameworks) I am told there are 3 ways this can be accomplished.

The first, and most obvious to me, is using AJAX.

The second can be done by using socket.io.

My question is what is the third?

Does anyone know of a good resource that explains these three ways?

You can use any one of this method:

Message Passing, Shared memory, RPC and java RMI.

To know more about Process Communication

Details about client and server Communication follow these links

Just to answer your broad question:

DDP, as being used by Meteor