Couchbase allows you to partition your data into separate containers or namespaces. These containers are called 'buckets'. Couchbase will keep item storage separated for different buckets, allowing you to perform operations like statistics gathering and flush_all on a per-bucket basis, which are not workable using other techniques such as simulating namespaces by document ID-prefixing.
Couchbase Server supports two different bucket types, Couchbase and memcached. For a full discussion of the major differences, see Section 10.6, “Buckets”.