Text is just to paste some plain ol' text. It doesn't attempt to execute it.
I sure hope they're emulating a CPU. I wish we had a kernel good enough to sandbox untrusted native code, but as far as I know there isn't one. OpenBSD had at least two local root exploits that we know of, and Linux (which they claim to be using) is worse off.
What is the difference between Rhino and Spidermo javascript?
They are different implementations of standalone JS. Rhino is an interpreter written in Java while SpiderMonkey is a JIT compiler written in C or C++, I'm not sure which sorry.
Ideally there would be a CommonJS option as well (using narwhal).