There is only one file type that uses the .NRO extension:
Nintendo Switch Executable File
An NRO file is an executable file used by the Nintendo Switch video game console or Switch emulator, such as Yuzu or Ryujinx. It contains compiled code for an application or game. NRO files may also store assets for Homebrew Launcher, such as a 256x256 JPEG icon and metadata.
You can use a homebrew application or game (application or game developed by a user) on a Switch running Homebrew Launcher by copying the NRO executable file to the switch/ folder in the root of the Switch micro SD card. NRO files are named with the same name as the application or game it contains. The files are scanned by the Homebrew Launcher and display them in the menu.
Updated: May 22, 2019