iPAD: The rounded button? (Is it a power button?)

Hi,
I just got mini iPad.

I still got a lot of confused for the followings:
1) When I did not do anything, the scree becomes “Dark”,
2) Then, I press “a rounded button”, then I “scroll the screen”; the “passcode screen” will appear

My Question: What is “the rounded button”; is it a “power button”? (If it is not a power button, where is the power button?)

Thanks,
tjie