Android
For Android 4.0 and above, press and hold the Power and Volume down buttons simultaneously. You should hear a shutter noise and see the screen flash. A notification will appear at the top of the screen, from which you can access your screenshot.

iOS
To capture a screenshot on an iPhone or iPad, press the Home button on the front and the Sleep/Lock button on the top of the device simultaneously. The screen flashes white when done correctly.
Your screenshot will save to the Photos app under Saved Photos or Camera Roll, depending on your device.

Mac OSX
Press Command + Shift + 4 and capture the area with the crosshairs.
The screenshot is saved to your desktop as 'screenshot date time.jpg.'

Windows

While viewing your screen, press Alt + the Print Screen key, usually located in the top-right of your keyboard. If you'd like to capture the whole desktop rather than just your browser, press the Print Screen key without pressing Alt.

Open Microsoft Paint by going to Start > Programs > Accessories.
Press Cntrl + V to paste the screenshot into Paint.
Open the File menu, and select Save as.
Name the file, select .jpeg as the file format, and save the file to your desktop.