Difference between alert log and archive log in oracle databases

I don't know anything about oracle databases. Actually I'm asking this question for my friend (He currently doesn't have access to internet!).

He wanted to know that what's the difference between alert logs and archive logs in oracle databases? Actually here is the scenario: There was a database for which auditing by alert logs is disabled. Someone came and truncated some data from this database. My friend didn't notice that because auditing by alert logs was disabled. But an oracle expert managed to find information about those transactions by looking at archive logs.

Now what I want to know is that: What type of information stored in each of alert logs and archive logs? and under what conditions?

I searched documentations of oracle but unfortunately couldn't find anything. The only thing I found was mandatory auditing. But in mandatory auditing information about transactions and changes to the database isn't stored.

Thanks.