Ubuntu e seus logs

/var/log/auth.log - stores messages about authentication, including opening of sessions, etc.
/var/log/daemon.log - Log file for daemons that use it; most often daemons will also log to another file too. Some (like Apache) don't seem to use it at all.
/var/log/kern.log - Kernel messages (including boot messages).
/var/log/lpr.log - I assume based on the filename that this relates to printer messages.
/var/log/mail.log - Messages relating to mail, etc. (also mail.info, mail.warn and mail.err)
/var/log/user.log - Not altogether sure what this is.
/var/log/uucp.log - This will probably be empty unless you're into using sz or anything like that.
/var/log/syslog - Stores all other messages. This will be where you'll find your cron jobs, unless Ubuntu does it differently - in which case you'll probably find it in /var/log/cron.log.

You can find these yourself in /etc/syslog.conf, in case Ubuntu differs in that regard.