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.

CouchDB is designed for document-oriented applications. A typical real-world document oriented activity, if it weren't computerized, would consist mostly of physical paper documents. These documents would need to get sent around, edited, photocopied, approved, denied, pinned to the wall, filed away, buried in soft peat for six months, etc. They could be simple yellow sticky notes or 10,000 page legal documents. Not all document-oriented applications have real world counterparts.

Some examples of document-oriented applications:

• CRM
  
• Contact Address/Phone Book
  
• Forum/Discussion
  
• Bug Tracking
  
• Document Collaboration/Wiki
  
• Customer Call Tracking
  
• Expense Reporting
  
• To-Dos
  
• Time Sheets
  
• E-mail
  
• Help/Reference Desk