On the server, you define a realtime queries using
Meteor.publish, so you can publish data from any source: your database, IMAP, or the forklifts in your warehouse.
On the client, you call
Meteor.subscribeto connect to a publication endpoint, receive the data, and listen for updates. The data is inserted directly into Meteor's client-side cache, so you can bind your HTML templates to it and they'll update automatically.
Meteor's DDP implementation handles the rest. It intelligently polls your database to pick up changes and push them down to the client. And it can simulate your model code on the client, so your users see their screens update immediately without waiting for the network.
DDP is a very simple protocol. It's just a codification of what you'd inevitably write yourself if you built this by hand. Because it's so simple, it's also general. DDP can work with any database, framework, or programming language, and it can work on servers, clients, and mobile devices. In fact, you can call
Meteor.connect to connect to any DDP server -- such as a Meteor app written by someone else! -- and make realtime subscriptions to any data it's publishing.
The features in this release have been promised in red boxes in the documentation since Meteor 0.0.1. We're very happy to have executed through so much of our roadmap and delivered them to you.