MongoDB is document database that supports range and field queries.
A single server can run either standalone or as part of a replica set. A "replica set" is set of mongod instances with 1 primary. Primary: receives writes, services reads. Can step down and become secondary. Secondary: replicate the primary's oplog. If the primary goes down, secondaries will hold an election. Arbiter: used to achieve majority vote with even members, do not hold data, don't need dedicated nodes. Never becomes primary.