Windows PCs
Capture the Whole Screen
Print Screen will copy the contents of your entire screen to the clipboard (where anything "copied" goes, before you "paste" it).
This means you'll have to open another application, like Paint, Photoshop etc, and paste (Ctrl + V) then save the image wherever you'd like.
Just the App you have Open
Pressing Alt + Print Screen will capture just the contents of the window you have open/selected.
You'll still need to "Paste" it and save it somewhere though!
A Part of the Screen You Select (or either of the previous options!)
Snip & Sketch can actually do all of the above, so if you're not sure, stick with this one! It's installed on all Windows Computers by default, and you can find it in the Accessories folder, but if you remember my other Windows trick, just press the Windows key and start typing Snip & Sketch.
To use the tool, just click "new", then use the mouse to select whatever part of the screen you'd like to select. Then, annotate/edit or save that snip!
MACS
Capture the Whole Screen
Macs are actually significantly better than PC's at this, because once you press those keys the screenshot is already saved on your desktop as the date and time it was taken.
Just the App you have Open
Pressing command + shift + 4, then space-bar will change the cursor that you'd otherwise click and drag into a window selection tool. Now just click on the window/app you want to capture as a screen shot, and that'll be saved to the desktop.
A Part of the Screen You Select
Pressing command + shift + 4 will allow you to select a portion of the screen by clicking and dragging, which will then be saved to your desktop as an image.
How Is This Useful?
Of course you can use this for many different applications, I use it at least once a day, and extensively when I write these tips! It also helps me understand what's wrong with your computer when you contact me if you're specific about the issue, and you can't get much more specific than a screenshot.