When you double-click a file, a default app launches to open it. But what if the app that runs isn't the one you want, and you'd rather have another program handle the task of automatically opening that file format?
Taking a screenshot on a Mac is a simple process. However, on a Windows PC, things are a little more complicated. On Windows 10, you can take a screenshot of the entire screen by pressing the PrtScn (PrintScreen) key, though you'll have to paste the image into an app afterwards (like Microsoft Paint, where it can then be saved out as an image file, or Word, where it will appear in your document).