BigCouch is an interesting effort to integrate clustering into CouchDB, making it more like Riak.

FORBES: What You Need to Know About NoSQL Databases

CouchDB queries are views, which are themselves documents in the database that specify searches.

FORBES: What You Need to Know About NoSQL Databases

In a document database, such as CouchDB or MongoDB, documents are key-value pairs.

FORBES: What You Need to Know About NoSQL Databases

CouchDB goes so far as to serve as a web application framework.

FORBES: What You Need to Know About NoSQL Databases

CouchDB and MongoDB are essentially single-headed databases, and from that standpoint, availability is a function of how long you can keep the hardware running.

FORBES: What You Need to Know About NoSQL Databases

RESTful interfaces: CouchDB and Riak are unique in offering RESTful interfaces.

FORBES: What You Need to Know About NoSQL Databases

Now that Cloudant has announced that it is merging BigCouch and CouchDB, we can expect to see clustering become part of the CouchDB core.

FORBES: What You Need to Know About NoSQL Databases

CouchDB and MongoDB both store documents in JSON format, where JSON is a format originally designed for representing JavaScript objects, but now available in many languages.

FORBES: What You Need to Know About NoSQL Databases

Like CouchDB, Riak incorporates MapReduce to perform complex queries efficiently.

FORBES: What You Need to Know About NoSQL Databases