I don't think that's a very thorough answer to the OP's question.
You are just going to have to search for articles on learning to use Command Line Interface. If you've ever used DOS or MS Command Prompt then it's something like that.
Linux has an emphasis on CLI, you can do almost anything from the command line and most things are more efficient from terminal than a graphical approach.
You may want to play around with a Linux Live CD on a desktop/laptop as xterm uses busybox (limited default commands, lots stripped out). You should also know that there is a certain amount of Tab completion (tap the tab "button" a couple times).
ROXterm and packages such as bash3 may also be useful.
As OS 2008 is based on Debian and Ubuntu is based on Debian (Mer is merging with Ubuntu MID) the common code base and structure allows you take advantage of forums such as ubuntuforums.com for tutorials.
The basic things you should learn are: cd - change into a directory. For example cd /home/user/ ls - list contents of the current directory/folder cp - copy a file. cp -R copy a folder mv - (be careful) move a file or directory ssh - connect to a linux machine. ssh root@192.168.1.100 for example scp - uses ssh to copy over a network. scp /home/user/file.txt root@192.168.1.100:/media/ df -h lists partitions and disk usage du -sh disk usage of directory. du -sh * disk usage of each file or folder within a directory. apt-get install package install software from CLI. (need to be root)
simple if u wanna compare it with ur pc, u'll find it somehow quite similar to the MS Dos as commands, with some deferences (such as case sensitive).
however, as the other gentlemen mentioned, u'll not need it always.. or let's say, u still can survive without it..
good luck..
I don't think that's a very thorough answer to the OP's question.
Yours isn't either! You do not even mention a single answer to the above question!
BASIC:
x-terminal is a program to display the command-line-interface (also referenced to as CLI or text-shell) in a GUI (graphical-user-interface) environment (thats the desktop you see and can click on).
DETAIL:
A shell is actually providing an interface to the system.
A shell providing a text-interface (ash, bash, ksh, ...) is called CLI and providing a graphical-interface (windows explorer, kde, gnome, ...) is called GUI.
In N900's case the provided text-shell is ash, that can be changed if needed.