What CouchDB is
* A document database server, accessible via a RESTful JSON API.
* Ad-hoc and schema-free with a flat address space.
* Distributed, featuring robust, incremental replication with bi-directional conflict detection and management.
* Query-able and index-able, featuring a table oriented reporting engine that uses Javascript as a query language.
What it is Not
* A relational database.
* A replacement for relational databases.
* An object-oriented database. Or more specifically, meant to function as a seamless persistence layer for an OO programming language.