What CouchDb is
- A stand-alone document store, accessible via XML REST.
- Ad-hoc and schema-free with a flat address space.
- Distributed, featuring robust, incremental replication with bi-directional conflict detection and resolution.
- Query-able and index-able, featuring a table oriented reporting engine with a simplified formula 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.