DDP is very simple. Think of it like REST for websockets. Here is the project page: https://www.meteor.com/ddp and here is the spec: https://github.com/meteor/meteor/blob/devel/packages/ddp/DDP.... If you have any questions, post them on forums.meteor.com and we'll try to help you out.

It's not "tightly coupled" to MongoDB; Mongo is simply the only database that livequery (https://www.meteor.com/livequery) has implemented support for at the moment. There are community projects for MySQL and PostgreSQL, as well as partially complete drivers for Redis and Elastic. Official (My)SQL support is on the roadmap and will probably appear later this year.