How do you put a picture on top of another?

I'm trying to 'stack' two pictures into one picture using Photoshop CS2. I want to put one image on TOP of the other NOT blend in. I know it's possible, but I dont have the slightest clue how to go about this, and I cant find anything about it on the internet. Please dont be vague, and tell me how to do this properly. Thanks in advance!