MongoDB is an open-source, high-performance document database that supports OS X, Linux, Solaris, Windows and FreeBSD. It consists of mongod (the database server), mongo (the shell), and mongos (the sharding router). The document discusses how to install and start MongoDB, use replication, and monitor the database using tools like mongostat, iostat, the web console, and serverStatus command. It also covers backups using mongodump and fsync + lock, and encourages downloading and testing MongoDB.