Yes, you still need backups, if somebody screws up the master node and it replicates, then your replica sets won't be able to save you.

MongoDB covers it quite well in their blog: mongodb.com/blog/post/backup-vs-replication-why-d…

and in their docs: docs.mongodb.org/manual/core/backups

Yes, like Jan says you have to back it up just like any other data. There are a number backup services out there as well.

But something that fascinates me (and I haven't tried this yet in practice) is whether or not you can use the slaveDelay and hidden replica set config parameters to effectively backup your system using MongoDB's replication mechanism.