This is a work in progress.
Part 1: Important Log Files
Problems with system stability, hardware related problems, boot related problems.
syslog and dmesg
The system log typically contains the greatest deal of information by default about your Ubuntu system. It is located at /var/log/syslog, and may contain information other logs do not. Consult the System Log when you can't locate the desired log information in another log. It also contains everything that used to be in /var/log/messages.
If you want to review messages from the current day, use command:
cat /var/log/syslog|less
You can scroll up and down to look for issues, and do regex search.
You can actively monitor the syslog to watch for errors while you work or perform an activity which typically creates a problem;
tail -f /var/log/syslog
Keep that running in a terminal window while you work. Messages and errors will appear in real time.
They call dmesg the kernel ring buffer utility. In this case, dmesg is an actual command you execute which displays kernel log data. This information is also sent in real time to syslogd or klogd, when they are running, and ends up in /var/log/messages. The dmesg command captures boot-time messages from before syslogd. Type:
dmesg|less
Again, the pipe - less is used to buffer it so you can scroll up and down or perform regex search. Use dmesg to see what occurred during system boot. Things go wrong such as a file system failing to mount or a device driver failing to load, and these events will show up in dmesg.