CouchDB 2.0 is a direct descendant of CouchDB 1.x. The API is 99% compatible, same community working on it, you can replicate between them and so on.
I haven't used Couchbase, but I understand besides the "Couch" prefix and that CouchDB's original author working there for a few years, it doesn't have much in common with CouchDB project.
Well Couchbase Lite is largely compatible as far as I can attest. Not sure how it is with Couchbase proper, but I was under the assumption that it is API compatible to CBL.
I haven't used Couchbase, but I understand besides the "Couch" prefix and that CouchDB's original author working there for a few years, it doesn't have much in common with CouchDB project.