Tails is
Sonic the Hedgehog's two-tailed fox sidekick in the
Sonic the Hedgehog universe. He's described as young, eager, intelligent, and anxious for action. He's about as fast as
Sonic and can do his
spin dash maneuver. One bit of trivia is that "Tails" is actually his nickname. His actual name is
Miles Prower (say it real fast).
Miles Prower was born on Westside Island, an isolated corner of Sonic's world. The other animals on Westside Island always made fun of Tails because of his unique two-tails oddity, and as a result the fox grew up with a severe case of shyness. However, on the day that Tails first encountered Sonic the Hedgehog and his immense speed, he hid from the blue blur and stumbled upon the hedgehog's custom biplane, the Tornado. Tails fell in love with the aircraft and soon after got to know Sonic. Ever since he's been Sonic's best pal and pilot.
One of Tails' most unique abilities by far is his ability to use his two tails to fly for short periods of time. Eventually he becomes tired and, panting, falls to the ground. Tails uses this special little technique not only for his own benefit, but to carry Sonic around as well through the skies (even occassionally fighting a boss in midair). He also uses it to propel himself when at top speed, zipping over the ground at a speed approaching Sonic's. Tails is also unusually intelligent for his age and his skill with devices is equal to that of Dr. Robotnik's. Tails can gather the seven Chaos Emeralds and become Super Tails, thereby becoming invincible.
Tails first appeared in Sonic the Hedgehog 2 as the second player. In one-player mode he was controlled by the CPU and followed Sonic around. In Sonic the Hedgehog 3 he became a fully playable character and was able to do his flight move as well as swim in deep water. Tails also starred solo in two Game Gear games, Tails' Sky Patrol and Tails' Adventures.
In Sonic Adventure 2 (and Sonic Adventure 2 Battle) Tails rides around in his custom-built mech that can jump, hover, and shoot missles with a laser-guided tracking system. He's quite the mechanic, it turns out.
A list of Tails' game appearances follows:
Tails originally had Knuckles the Echidna's role in the Sonic the Hedgehog saga. He was to be the protector of the Master Emerald in the Hidden Palace Zone in Sonic the Hedgehog 2. However this plot thread was dropped when the Hidden Palace Zone was removed from the game. If you can find a beta ROM of the game floating around online, you can sample the level and see the Master Emerald for yourself.
Tails is the #2 man in the Sonic the Hedgehog series, so it's pretty much guarenteed that we'll see him again. His loyalty and dedication to Sonic are second to none.
Some info from http://www.sonichq.org/general/profiles/index.php/top.a/bottom.m/category.sega/type.character/index.10