What is the exact difference between mongod and mongos

Question

While I was looking into the MongoDB documentation, I found two different terms mongod and mongos.

While starting the server, I am using

Answer 1

The core components in the MongoDB package are: mongod - the core database process; mongos - the controller and query router for sharded clusters; and mongo the interactive MongoDB Shell.

For more information http://docs.mongodb.org/manual/reference/program/