The TurboGrafx-16 is a console created by NEC. In Japan, it was known as PC Engine. While it enjoyed a relative success in it's homeland, it was ignored in the US due to bad marketing and was not properly released in Europe. It later received a CD-ROM expansion, an upgraded model supporting both cards and CDs as well as a handheld version of the console.
There was also an upgraded model called the SuperGrafx in Japan where it had 5 exclusive games but could not run CD games.
For more details on this program, please visit the Mednafen page.
All info about this program can be seen here.
Actually a multiplatform emulator, and probably the best choice for emulating the TurboGrafx-16. A command-line program, it supports both Hu-Card and CD games, as well as games which use the Arcade Card expansion.
A better choice if you don't like to tinker with command-line programs. It still doesn't support the Arcade Card expansion, however.