How do I backup Exchange 5.5/understand the DBs/recover a mailbox?

This question is phrased like that because these, and a whole bunch of other really important questions, are all answered in a white paper that Microsoft wrote a long time ago. It is called the Exchange 5.5 Disaster Recover Whitepaper, but it is really misnamed - it does cover DR, but to be able to understand correctly how to DR your server you also need a thorough understanding of how to back it up, how the databases and transaction logs work, and thus how to recover a single mailbox from a backup tape.

This whitepaper is required reading for every Admin of a production Exchange 5.5 server:

http://www.microsoft.com/technet/prodtechnol/exchange/55/support/exdisrec.mspx

Once you've read it, please feel free to post clarification questions on the forum.