What exactly do people do when they're on their iPhones?

Inspired by this question.
So many people I see nowadays are always walking down the street with iPhones, Smartphones, or any handheld device like that. People are constantly focusing their eyes and straining their necks to see what is going on on the tiny little screen. I don’t have one, so I want to know what the hell is going on? Can I know or is it a secret? Are they discovering all of the secrets of the universe? Am I going to turn into “one of them”?
I once observed a group of four friends sitting at a table once in the cafeteria. Nobody was saying anything to each other; all four of them were just messing with their phones.