Another handy guide is http://freeengineer.org/learnUNIXin10minutes.html .

Linux is built on top of UNIX, and many of the commands you will come across in this forum are UNIX commands.

For me as a beginner, the most confusing thing was the idea of "permissions" in UNIX. Whatever I tried to do, I would get an error message: "You do not have permission to do this." It took a while before I understood that I could give myself the permission by using chmod and chown commands. That's not the way you get permission in the big, bad world outside.

STANDARD WARNING. Using the command line, you can end up with a Maemo device that does not work. It could freeze up, it could reboot over and over again, it could go completely blank. To get it working again, you might have to reflash the firmware. Until you are comfortable with the flasher, you are better not to use X-Terminal. You also need to be comfortable with taking out the battery, because that might be the only way to turn off your device before reflashing.