Learning the command line teaches you Unix, it doesn't really teach you about Mac OS X.
There are other posts about learning the command line. You should even be able to see some on the right hand side of the web forum. And you can search for Terminal and most likely find similar posts. There have been pointers to online Mac OS X Terminal command tutorials.
Go to a bookstore and browse just about any book on Unix/Linux command lines stuff. Especially books on "Bash" (the default Mac OS X shell), and other books on Unix/Linux scripting. While not everything will be the same, 80% of what these books talk will apply to Mac OS X.
You can find useful posts at MacOSXHints.com which frequently has posts about doing things via the Unix side of Mac OS X.
Finally the Terminal is nothing, it is just a bit of software that displays the output from the shell, and programs run by the shell, and passing input from the keyboard to the shell and programs run by the shell. Besides that the Terminal doesn't really have any significant power.
Use the "man" command to find out information about various commands ("man man"). Use "apropos" to find possible commands that will do what you want ("man apropos").
Most of the commands that the bash shell will execute will be found in /bin, /usr/bin, /sbin, /usr/sbin, and /usr/X11/bin. If you have installed XCode developer tools, you will find more commands in /Developer/usr/bin, /Developer/usr/sbin, /Developer/Tools. Additional progarm may be found in other locations, as a command is just an executable program, or executable shell script, or a built-in command within the shell (bash).