Some kind of command window

[Zylstra]

On my Mac, sometimes, when it freezes, I try to remember the Restart key combination (APPLE,CTRL,POWER) but end up getting some other combination. And then this box comes up that looks like an old DOS prompt.

It looks like this

 >

(yeah, really exciting, described with one text character and a space)
You can type stuff in, but when you press ENTER, it clears again.
Any idea of what it is?

(If you search Google, and found the answer, tell me what you searched for)

[Zylstra]

Update:
This happens when you press the apple key, then the keyboard power button.
Shows in a white box with a black border and black text.