Couchbase Server is one product, but it’s made up of a combination of different components that work together in order to produce server product, including memcached and the spidermonkey JavaScript engine, and more recently the CouchDB engine for storage and index creation.
An article covering more of the detail is available here:
http://blog.safaribooksonline.com/2012/03/01/the-technology-behind-couchbase/