How to take a MacOS screenshot
Two options: capture entire screen or just a portion of it.
Where will the screenshot be?
Usually the screenshot is automatically saved to your desktop but if you installed Dropbox you may have a “Screenshots” folder in your Dropbox that files will appear in. Also, your screenshots may appear in your downloads folder.
How to take a screenshot of your entire screen
- Press Shift-Command (⌘) 3
How to take a screenshot of a selected portion of your screen
- Press Shift-Command-4. The pointer changes to a crosshair.
- Move the crosshair to where you want to start the screenshot, then drag to select an area. (tip: hold Option. Shift or Space to modify the way the selection moves.)
- When you’ve selected the area you want, release your mouse or trackpad button. To cancel, press the Esc (Escape) key before you release the button.
crosshair: 
How to take a screenshot of a window
Press Shift-Command (⌘) 4 and note that your pointer changes to a crosshair. Press the Space bar and pointer changes to a camera. Move the cursoe over a window to highlight it. Click your mouse button to finish.
iOS 11 and screenshots
October 2017: taking a screenshot is just a bit different than in previous versions of iOS. Users initiate by pressing two buttons — they will still see the screen flash, but beyond that, you have many new editing options.
There is a new Screenshots album, and Screenshots floating interface.
Additionally, when you take a new screenshot (or series of screenshots in a short period of time), the snap minimizes itself in the bottom left corner of the screen. From here, you have four options:
- swipe it off-screen to save it to your Photos
- Leave it alone, and the thumbnail will simply be in Photos library after a few seconds
- long-press on the thumbnail to send it as-is via the “Share Sheet”
- tap on the thumbnail to enter Markup mode (edit mode) — without saving it to your Photos library. (this is the most useful feature of the floating interface)
This video shows how to markup a screenshot:
