This document provides an overview and introduction to key MongoDB concepts including: - Replication which allows for failover, backups, and high availability through asynchronous replication across replica sets. - Sharding which provides horizontal scalability by automatically distributing and balancing data across multiple shards in a cluster. - Consistency and durability models including eventual consistency and different write acknowledgement options for ensuring data is safely written. - Flexibility in data modeling through embedding and linking of related data as well as the use of JSON which maps easily to objects.