If you want to take a snapshot of what is displaying on your phone screen. Press the home button and power button simultaneously. A snapshot of your screen will be taken and stored in a folder called screenshots. You can find it using your preferred photo app.
This tip might also work on Android phones other than Samsung.
***Edited to add labels***
Another way to take screen shots is to lay your whole finger sideways on the screen and swipe it from one side to the other. I find it easier than using the buttons, because I so often found my fingers out of sync and struggling to get the screen shot without either going to my home page, or turning the power off...