After failing to make a real dent against the NES, Sega decided just to top them. If Nintendo was dragging their feet to a 16-bit system (the arcade standard at the time), then Sega would beat them to the punch with a console based on its System 16 arcade board.
For the most part, it worked. This was helped by some of Nintendo's U.S. policies being ruled as anti-trust violations, by some developers supporting Sega due to them being a lot more lax , and their first truly successful hit known as Sonic the Hedgehog. So the Genesis was a hit, selling 35 million systems (with miniaturized versions and handhelds still on the market today.). There were also the Sega CD and 32X add-ons, but they were commercial flops.
Like the NES and SNES, the Genesis could expand through chips on the carts. One was the Sega Virtua Processor, which functioned like the Super FX chip on the SNES. Unfortunately, incorporating it was a lot more expensive than an SNES chip, and only the port of Virtua Racing used it. The system was heavily marketed for its ability to render objects faster than the SNES, a feature for which the Sega marketing division coined the term "Blast Processing".
It also has a 2nd CPU, a Zilog Z80 running at 3.58 MHz, used for the Sound chips's CPU and Master System play back.
8 KB of extra RAM for backwards compatibility with the Sega Master System (although that requires an adapter).
Games ranged from 512 KB (most launch games) to 5 MB (Super Street Fighter II). Keep in mind that these were advertised by their bit size, not their byte size, so they would be listed as 4 megabits to 40 megabits.
modded for stereo sound (the chip's standers can only do Mono)
Addons and peripherals
Power Base Converter: An add-on which allowed the Genesis to play Sega Master System games, either of the cartridge or the chip variety, and included support for the SMS's 3D glasses. This was initially marketed for the first model Genesis, but a small quantity was made for the redesigned, compact Genesis But only in Europe. Also, it can not play SG-1000 games (or Master System games that uses the system's video modes like F-16 Fighter Falcon) or use it's Japan only FM chip (the YM2413, which was also used on the MSX under the name MSX Music and was cloned by Konami as the VRC7 chip for the Famicom in Japan) unless the unit is modded. It also won't work with a 32X unless it is modded.
Sega CD: A CD-based add-on which would allow the Genesis to take advantage of a higher-capacity storage medium, enabling features such as Full Motion Video and Red Book CD sound. Unfortunately, the Genesis' own processing power wasn't quite enough to take advantage of these features to the fullest. The add-on actually sold decently, though the critical reaction led many to believe otherwise.
A 2nd 68000 chip running at 12.5 MHz, the main 68000 chip becomes the sound chips's CPU.
512 KB of main RAM and 256 KB of video RAM.
64 KB of sound RAM.
16 KB of CD drive cache.
8 KB of back up RAM, with memory cartridges going at 128 KB
Same as the Genesis but has a extra chip the can do scaling and rotation effects like the Super NES's Mode-7 chip (the Super NES has 2 PPUs, 1 for modes 0 to 6, and the other for mode 7) with the DPS1 chip and playing FMV video.
16 bit 8 channel PCM chip running at 32 KHz (44.1 KHz for CD-DA), also it's own CPU running at 12 MHz.
Sega 32X: Originally conceived as the Neptune, a cartridge-based 32-bit system to go with Sega's later CD-based system, the Sega Saturn, the add-on boasted two 32-bit processors and primitive 3D graphics capabilities, and was marketed as an opportunity for consumers to get a head start on the 32-bit generation. Unfortunately, both consumers and developers knew that the superior Saturn was just around the corner (even though Sega themselves believed the 32X and Saturn could co-exist, with casual gamers gravitating towards the cheaper 32X while the Saturn was reserved for the hardcore crowd), and titles for the add-on were few and far between. Some previous Sega CD games were also re-released on the 32X to take advantage of the system's improved processing.
2 Hitachi SH-2 chips, just like the Sega Saturn, but unlike the Saturn, the chip are a bit slower and are running at 23 MHz. each.
256 KB of main RAM and 256 KB (128 KB X 2) of video RAM.
256 KB of sound RAM
2 frame buffers with 2 layers (sprites and backgrounds) each (4 in total) and can be set up as just backgrounds or a large amount of sprites or ect.
32,768 Colors, no on screen limits.
50,000 sprites with their blocks going up to 512 X 512; Polygons like the Saturn are done with sprites, if all 4 layers are sprite layers, it can go up to 200,000 sprites.
Stuff like Scaling, Rotation and 3D Engines are done with software with said software running on the 2th SH-2 chip.
Screen resolution however is still the same as the Genesis.
Virtua Fighter 2: Actually a 2D version of the original 3D game with a lot of the content stripped out. A better version of the game, with actual 3D graphics, was later made available on the Sega Saturn.
Virtua Racing: Notable for having the Sega Virtua Processor microchip, Sega's answer to the SNES's Super FX microchip, making it the second of two 100% real time 3-D video game available for the system. An upgraded version, Virtua Racing Deluxe, was made for the 32X, possibly to compensate for the fact that the original game won't play on a 32X equipped Sega Genesis.
Long Runner: It was launched in 1988, and it wasn't formally discontinued until 1998. But, there are still versions of the console on sale today, and there were actually a few new games released for it in the last decade, the most recent of which came out in 2011.
Polygon Ceiling: Towards the end of its life in the mid-nineties Sega attempted to create (at least the illusion of) 3D games on the system, such as with Sonic 3D: Flickies' Island, Vectorman 2, certain levels in The Lost World and others. While the graphics were ambitious for a 16-bit system, the gameplay tended to suffer as a result. Virtua Racing managed to break through it thanks to using the Sega Virtua Processor chip to allow real time 3D graphics, but the added cost of this chip (which skyrocketed the games cost to 100$) kept anymore games with the SVP from being made.
To add insult to injury, due to relying on certain hardware, Virtua Racing was incompatible with the Model 3 Genesis (unless you mod it) and pretty much all of the unofficial clone systems. So if you bought the game years later and happened to own a Model 3 system, you were SOL.
Product Facelift: The Genesis went through the most redesigns of any video game console in history--first, there's the model 1, which also has a link port (meant for the cancelled Sega Meganet) in the very, very earliest models, the more famous, streamlined model 2 Genesis, the Sega CDX which was a clever (but expensive) hybrid of the Genesis and Sega CD, the Model 3 Genesis from Majesco, which was as big as the controller, and then there's the Sega Meganet/The Sega Nomad, both of which are literally portable Sega Genesis consoles! There is even a licensed version of the Genesis released in Europe, the AtGames Sega Mega Drive 20-in-1 Game Console, which contains 20 games built into the console and has Region Coding fully unlocked, is even smaller than the Model 3 Genesis, and has unofficially been dubbed the "Model 4 Genesis".
Scapegoat Ad: Sega's famous anti-Nintendo commercials are fondly remembered by many a nostalgic Genesis fan. Unfortunately, when Sega began to be brought down by their failing add-ons, Nintendo took the opportunity to do their OWN Take That to Sega in the commercial for Donkey Kong Country, which advertised that such a technically ambitious game was NOT a Sega game and didn't need a CD or 32X adaptor to be played. Now the ads are Hilarious in Hindsight.