The Sega CD, known as Sega Mega CD in Japan and Europe, was a 16-bit video game console that was popular in the early 90's. It was one of the first to use CD format games.
In the late 1980's, 16-bit systems were gradually taking over the home computer and arcade machine business, with computers like the Commodore Amiga and Atari ST, and Sega's System 16 based arcade games. Meanwhile, Sega's 8-bit home console, the Sega Master System, wasn't doing at all well. Nintendo, with it's own 8-bit console, ruled the markets both in Japan and the west.
In this situation Sega decided to create a new, 16-bit home console, based on it's System 16 arcade game technology. The Sega Mega Drive, the first true 16-bit home console, was released in Japan in 1988. It was brought to America, renamed Sega Genesis (because Mega Drive was already trademarked by another company), in 1989, and to Europe (as Mega Drive) in 1990. Nintendo quickly rose to the challenge and released their Super Nintendo Entertainment System in late 1990., but the Mega Drive was still very successfull in the early 90's.
The Sega Mega CD, as it was titled in Japan and Europe after the Mega Drive, was released shortly after Nintendo began to retake the market with the SNES, late 1991 in Japan and in 1992 in America, titled simply the Sega CD (Europe didn't see it until 1993, and it was never very successfull there.) It was one of the first CD consoles to hit the home markets, though not the first, as NEC's PC Engine (or TurboGrafx-16 in the West) already had an optional CD module.
The Mega CD was an add-on for the Mega Drive and was plugged into the original console. This meant that that the graphical quality of the games remained pretty much the same. However, the CD format had considerable advantages when compared to the old Mega Drive cartridges, most importantly much more storage space. This made it possible include, for example, full motion video, voice acting and CD-quality music. And even if the quality of the graphics was the same, there could be much more them.
Alas, the Sega CD never saw as many classic games as, say, the SNES. There are a few worth playing, though. These are some of my own favourites:
Lunar: The Silver Star and Lunar: Eternal Blue
These are two classic fantasy RPGs developed by Game Arts and brought to the West by Working Designs, the company that brought us Popful Mail. Many now know them best from their PlayStation remakes. The games feature beautiful anime cut-scenes and some great music.
A cyberpunk adventure game by Konami, released in 1994, created by none other than the legendary Hideo Kojima.
Mansion of Hidden Souls
A horror adventure game published by Sega in 1993. It is very short, but features nice prerendered 3D graphics.