Excerpt from Mongo DB in Action – “Few databases identify themselves as document databases. The other well-known document database apart from MongoDB is Apache’s CouchDB.
CouchDB’s document model is similar, although data is stored in plain text as JSON, whereas MongoDB uses the BSON binary format.
Like MongoDB, CouchDB supports secondary indexes; the difference is that the indexes in CouchDB are defined by writing map-reduce functions, which is more involved than the declarative syntax used by MySQL and MongoDB.
They also scale differently. CouchDB doesn’t partition data across machines; rather, each CouchDB node is a complete replica of every other.”