Different operating systems:

1. macOS
2. Windows
3. Android device
4. iPhone, iPad or iPod
5. ChromeOS device
6. Linux

1. macOS

Hold down    and    and press  = Your Mac captures the entire screen and saves it as a file on the desktop.

Hold down    and    and press   = Use your mouse to draw a rectangle in order to specify what to capture – or press the spacebar and then click on something (e.g. a window) to capture it. Your Mac then saves it as a file on the desktop.

2. Windows

Hold down  and    and press   = Use your mouse to draw a rectangle in order to specify what to capture. Windows then copies it to the clipboard.

I guess it’s in the clipboard now. How can I paste it into a document or something?
Hold down    and press    = Windows pastes the screenshot (that is in the clipboard) into a document or image you are currently editing.

Where should I paste it? I just need a (graphics) file.

1. Start “Paint”
2. Paste
3. Save

3. Android
Hold down at once   and    for 1–2 seconds = The screen flashes white. Your device captures the entire screen and saves it as a photo.
Where do I find my screenshots?
Look for a Screenshots folder in your Gallery or Photos app.

4. iPhone, iPad or iPod touch

Hold down    then immediately press and release    = The screen flashes white. Your device captures the entire screen and saves it as a photo.

Which one is the 'Sleep/Wake' button?
It’s that isolated button which is either at the left (iPad), at the top (e.g. iPod touch) or on the right side (e.g. iPhone 7, 8, X, 11, 12, 13). You will find your screenshot in the photo app. 

5. Chrome OS device

Hold down    and press    = Your Chrome OS device captures the entire screen and saves it as a file into your Downloads folder.
Or
Hold down and press

6. Linux
Press    = In most Linux desktop environments (like KDE Plasma, Gnome, XFCE) this either takes a screenshot or starts a program for taking screenshots.