The Sega Mega Drive was a 16bit console that came out in Japan in 1988. The following year, it was released in North America as the Sega Genesis. The only 16bit competition at the time was the TurboGrafx-16, which was not very successful and as a result, early on, the Sega Genesis enjoyed a very successful run.
While the Genesis eventually lost momentum and sold much less than the SNES, there's never been a closer rivalry between fans. To this day, the Genesis VS SNES discussions come up quite often, and there's usually around an equal number of passionate fans on each side.
One interesting feature of the Genesis was that it was designed to be able to play the old Sega Master Systems games as well... kind of. While the system was designed to handle it, the cartridges themselves were slightly different and so an extra peripheral was needed to use the games with the system.
The Genesis probably has the most official versions of any console on the market. The Genesis 1 was a bulky beast, and eventually was replaced with the much slimmer Genesis 2. After the release of the CD add-ons, Sega also released and licensed several systems that combined both the Genesis and CD functionality. The Sega CD-X was a Discman-like system with a headphone jack and a Genesis cartridge slot on the back. It was marketed as a portable Sega Genesis and CD.
It also played CDs like a regular Discman and could run off batteries. Several other companies also released systems that could play Genesis games. For example, Majesco released the Genesis 3, and JVC released the JVC X'Eye.
Sega really ramped up the marketing for this generation. Things like the Sega "scream" and clever ads like "Sega does what Nintendon't" really pushed the brand. The Genesis was also where a lot of Sega's franchises started to shine. Prior to the Genesis, Sega didn't really have a lot of strong mascots. Alex Kidd was probably the only one, and I'm sure a lot of gamers today wouldn’t recognize that name. The Genesis, however, faetured Sonic and ushered in a ton of popular Sega properties including Ecco, Shinobi, Vectorman, Comic Zone, Streets of Rage, and Phantasy Star.
The Genesis had much more third party support than the Master System, but it still focused on its strong first party titles. Overall, the Genesis had over 700 games released.
In the later years of its life cycle, Sega started releasing add-ons like the Sega CD and 32x in hopes of revitalizing the systems sales
Like the NES and SNES the patent on the original Genesis has expired and tons of knock-offs are now being produced. Companies like Yobo and Innex release clone systems that can play genesis games. These systems are cheaply produced and the video/sound emulation isn't perfect but they are a good alternative for someone who doesn't want to spend the cash for an old system that they have to maintain in order to play games. The biggest selling point for these are that they just work. They don't have the lockout chip, and the pin connectors are new so you don't have to worry about cleaning every game. These systems go for $40-60, but usually also play either NES and/or SNES games as well.
Cartridges for the Genesis can still be found, and can be very cheap, but they are disappearing and are already much tougher to track down than NES and SNES games.
Many popular Sega Genesis games are also available on the Wii Virtual Console, some are also available on PSN and XBLA, and many popular titles have been released on collection discs by Sega including: Sega Smashpack (Dreamcast: 11 Genesis games), Genesis Collection (PS2/PSP: 28 Genesis games), Sonic's Mega Collection/Plus (Gamecube, PS2, Xbox: 7 Genesis games), Sonic's Ultimate Genesis Collection (PS3/Xbox 360: 40 Genesis games).