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.
Relational databases define a strict structure and provide a rigid way to maintain data for a software application. Apache's open source CouchDB offers a new method of storing data, in what is referred to as a schema-free document-oriented database model. Instead of the highly structured data storage of a relational model, CouchDB stores data in a semi-structured fashion, using a JavaScript-based view model for generating structured aggregation and report results from these semi-structured documents. CouchDB has been developed from the ground up with Web applications as the primary focus and has its sights on becoming the de-facto database for Web application development.