Screenshots usually require rooting and separate apps that could cost money. But Samsung built in a simple way for anybody to take a screenshot of what is on their screen.
It requires a combination of two buttons and is a little hard to get the hang of. First hold down the back button and then press the power button. Your Galaxy Tab will vibrate and a notification will pop up on the bottom of the screen to let you know that the screenshot was taken successfully.
You can view the screenshot through the built in file browser or a third party one if you prefer. Open up my files and navigate to the ScreenCapture folder on the root of the microSD card. An alternative method would be to open up the preinstalled gallery application and open up the same ScreenCapture folder.
Since the back button exits or goes backwards in the app, it can be tricky take a screenshot of open applications. After some practice you can perform it like a champ though!