Yes, CouchDB is an Apache project and Couchbase Single Server is a Couch One product. Except that they are almost exactly the same. Couchbase Single Server is already pre-built into easily installable rpms, it has GeoCouch integrated, but not a whole lot more compared to CouchDB trunk (Am I wrong?).

So as a developer you want to play around with Couch. Which one do you pick? ... Exactly. Aside from terminology there is basically a fork at the moment and I understand that CouchOne Server eventually will combine membase + CouchDB in it, and it is a great step forward but currently it makes it a bit harder. It is also not exactly easy to find a comparison of exactly which features are in which. So it is sort of a guess work.