In Oracle you have a number of online redo log files.
In an typical SAP installation you have two directories:
origlogA and origlogB
origlogA contains online redo log 1 and 3 and
origlogB contains online redo log 2 and 4.
In most installation you also have mirrlogA and mirrlogB, which the name tells, is mirrors of origlogA and origlogB.
Now, the online redo logs are circular,
so when 1 gets full oracle jumps to number 2.
At the same time oracle's archiver process starts to copy
online redo log number 1 to a file on disk.
This files are also known as archive redo logs.
In an recovery situation you might do with just the online redo logs,
but usually a combination of both online and archive redo logs is needed.
During database backup, you also backup the online redo logs
You also need to backup (backup twice then delete) archive redo logs.
the archive redo logs are your way back from the restored database to the last transaction of the database.
If one archive redo log is missing, you have to stop you recovery there and loose the rest of the transactions done in the database.
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