Jeff Benjamin writing for 9to5Mac:
After a screenshot is taken, you’ll notice that it doesn’t instantly appear on your Desktop as it did with previous versions of macOS. Instead, macOS Mojave has adopted an iOS-like system that places a floating thumbnail of the screenshot in the bottom right-hand corner of the screen, allowing users to interact with the screenshot before it is saved to the default location. By adopting this method, users can quickly markup, drag and drop, and share screenshots.
The above excerpt covers just one way the taking of screenshots has changed in macOS Mojave. Apple has made the process much more iOS-like, and added additional functionality, including annotations and screen recording. If you’re a longtime Mac user who needs to learn the new features, or if you want a thorough overview of how screenshots work on Mac, the 9to5Mac article will get you well on your way.