X-terminal allows you to run commands using a "command line" - ie you type in the commands instead of using a pointy-clicky gui interface. When you get to learn more about it you'll find that it is more powerful than a gui. Normally X-terminal runs using the default n900 user, "user". Rootsh allows X-terminal to run using the user, "root", this gives it FULL access to the whole system, which means you can easily bork your system if you don't know what you're doing.