A full screen capture shows far more than you intended. Cropping is as much about what you remove as what you keep.
Tab titles are the usual culprit. A screenshot taken to show a single paragraph also shows everything else you had open, and those titles are often more revealing than the content you meant to share.
Notification banners are worse, because they appear without warning at the moment of capture and contain message previews. It is worth scanning the top of the image before you send it anywhere.
Cropping removes those pixels entirely rather than covering them. That distinction matters: a black box drawn on top can sometimes be undone, while cropped away pixels are simply gone from the saved file.
Before you share a screenshot, look at its edges for the things that do not belong: the browser bar with your account, other tabs, a notification that slid in, a name in a corner. Pull the box in until only what you mean to show is inside it, and check the pixel size shown while you drag.
A screenshot saved as PNG stays PNG, so the text stays crisp and nothing is lost by cropping. If something sensitive sits in the middle of the picture where cropping cannot reach it, hide it with Blur or hide before you share.
No. Cropping discards pixels around the edge and leaves the rest untouched.
Yes, crop to the region you want and then resize to the pixel dimensions you need.
No, which is worth knowing given how much screenshots tend to contain.
Keep PNG for anything with text or interface elements, because JPG smudges the edges of letters. JPG is only worth it if a form insists on it.