Steam Overlay is a component of Steam that allows the user to use it on top of any game that you are playing. This won’t work for all the games, it can be only be used for Steam games.

The GeForce Experience in-game overlay enables you to access GPU-accelerated video recording, screen-shot capture, broadcasting, and cooperative gameplay capabilities. If the NVIDIA GeForce ...

Once you've got the overlay turned on, launch one of the games that you enabled it for. You should see a message appear in the corner of the screen telling you how to open the overlay — if you ...