Sonic The Hedgehog was created as a showcase game for the
Sega Mega Drive and debutted in 1991. Up until that time, Sega had sold the machine on the strength of their
arcade conversions, and had no
mascot to act as a face for the company. The character was created by
Yuji Naka and
Naoto Ohshima. Their goals were to exploit the
Megadrive's technological strengths (i.e. colours, speedy
hardware scrolling and
stereo sound); and create a timeless iconic character (there are echoes of
Mickey Mouse - who
Sega had previously licensed for the successful
Castle of Illusion - and
Felix the Cat in Sonic's appearance). He had enough "
attitude" to pitch to the American market while being
cute and aesthetically pleasing for the Japanese. (And they couldn't get enough of him here in
Europe -
Sega plastered his visage all over their
Sega buses and trains, and ran promotions with
Coca-Cola.)
The canonical Sonic games were 2D scrolling platformers. Each started in an idyllic coastal environment and progressed through 6 or more themed stages filled with the usual enemies, traps and obstacles as well as huge rollercoaster loops, rising tides, pinball machines, and whatever else the developers could throw at you. Each stage was split into 2 or 3 acts, the last of which culminated in a showdown with Dr. Eggman in one of his vehicles. Levels could raced through to earn a time bonus, or explored to allow the collection of gold rings (you would not die if you were hurt while carrying at least one ring - but you would drop all your rings), of which 50 gained you access to a special stage, and multiples of 100 would gain an extra life. The game was designed to be as self-explanatory as possible - no menus, no status bar, minimal text, one controller button - even the story was kept to a minimum.
I was going to include a list of all the Sonic games here, but you can see most of them for yourself at sonicfoundation.org (they miss a few, mainly coin-ops). You can also see the utterly shameful American box art for the later games. So far the blue spiky one has appeared on the Sega Mega Drive, Sega Master System, Game Gear, Mega-CD, Sega Saturn (just), Dreamcast, Arcade, mobile phone handsets, Neogeo Pocket Color, Game Boy Advance, Nintendo Gamecube and PC.
There are a couple of different versions of the game; later production runs featured ripples underwater in the Labyrinth Zone stage, and some other graphical tweaks.