Mongo Concepts and Terminology

See the Manual page for information on the following:

Config server. In a sharded environment, config servers store the metadata of the cluster. Each config server has a mongod process which stores metadata. Typically there are three config servers which have replicas of the same metadata (for data safety and HA). The config server mongod process is fairly lightweight and can be ran on machines performing other work.
Durability / [Journaling. Write-ahead logging for crash safety.
Member. A member (server) in a replica set.
mongod, mongos, mongo. MongoDB processes.
Object IDs. Mongo documents include an _id field in each document.
Oplog. High level log of operations used by replication.
Replication. Duplicating data on multiple servers for HA, safety, disaster recovery, and a bit of scaling. Sharding and replication are used together.
Sharding. The partioning / distribution of data among machines in a cluster. Each shard has different data. Sharding is the mechanism in MongoDB for building very large clusters. Note: we recommend you begin using MongoDB without sharding. It is easy to transition over to sharding later.
Shard, Chunk, Shard Key, Config Server. See the sharding intro page.

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