How do the MongoDB journal file and oplog differ?

I have recently started on MongodDb and I'm trying to explore on replica sets and crash recovery.

I have read it like journal file are write a head redo log file. oplog files are those where every write activity will be written to.

What is the difference between these two...? Do we have oplogs on both the master and the slave...? Please post any web links that shed some light on this area.