Luckily this is easy on Windows, Mac and iOS devices.
The simplest way to take a screenshot is to hold down shift+cmd+3 at the same time. You’ll then see a .PNG file appear on your desktop with the date and time of the screenshot. You can then share this with someone else using a file sharing service such as Dropbox or OneDrive or file it away as required.
Windows 8.1 and 10
Slightly longer process, but still nice and easy. Simply press the Print Screen (prt sc) button on the keyboard and then open Paint and click on the Paste icon. You’ll see your screenshot and you can then save it and again share it via Dropbox or OneDrive or save it into your Documents or Pictures folder.
If you have a Windows tablet, you can also press the Windows button and the volume down button on your tablet and you'll find a screenshot saved into a folder in your Pictures folder.
On iOS (iPhone, iPad, iPod touch) you can take a screenshot by pressing the home button and the lock button at the same time. The screen will flash and you'll then find the screenshot in the Photos app.
If you’d like more control over the area of the screen that you capture, on the Mac just press shift+cmd+4 at the same time and you’ll have a crosshair that you can drag out to take a screenshot of just that part of the screen. On Windows, open the Snipping Tool application and then you can choose from several options to grab just a part of the screen or a particular window.