I just put up this website, its ugly but working on the content, in particular I want it very easy to understand wtf it is and get started, would love to hear your feedback.
The server is really exciting, we mostly built it so we can reuse CouchDBs test suite however with it being so easy to install its becoming pretty invaluable
In the browser PouchDB will store its data in IndexedDB or WebSQL, in node its LevelDB, with the HTTP adapter you can use any product that implements the CouchDB HTTP Api protocol, currently this is CouchDB, Cloudant, PouchDB-Server and Couchbase Lite (previously TouchDB)
The important part is that it can only sync with a backend that implements the same replication protocol.