From Sonic Retro
Super Tails is Miles "Tails" Prower's Super State which is only seen in Sonic 3 & Knuckles. Tails simply glows and is surrounded by Super Flickies that attack any enemy on screen. Unlike Sonic and Knuckles, Tails requires all seven Super Emeralds to achieve this form, making it really more analogous to their Hyper forms, though unlike those forms, Tails still requires a Water Shield to maintain breath underwater. Additionally, Super Tails has the standard Super State powers of higher speed and invincibility.
Tails was given a new Super State in Sonic Heroes, thanks to Chaos Energy from Super Sonic. While the only physical change was a golden spherical aura (similar to Super Sonic's in Sonic Adventure), Tails received all of the modern abilities of a Super State. These include the ability to fly as well as the inability to brake. Additionally, Tails received the ability to use an attack similar to Sonic's "Light Attack". Fans were upset that this state didn't take on the original appearance of Super Tails and because of it, there is doubt in the minds of some that it is a Super State at all.
In the Archie comics, Super Tails was called Hyper Tails (which is itself a common misnomer for Super Tails, made common by Super Tails' similarity to Hyper Sonic and Hyper Knuckles). He also has a form called "Turbo Tails," which was analogous to Super Sonic and to Super Knuckles (who in turn was called "Hyper Knuckles"), resulting in some confusion.
When hacking into a prototype of the combined Sonic 3 & Knuckles, Tails not only has a Super form, but a Hyper form as well. His Super form is much more similar to Super Sonic and Super Knuckles, in which he gets super speed and invincibility, but does not have the golden Flickies seen in the final version. His Hyper form has the golden Flickies intact, showing that he originally had both forms planned. It is unknown why this wasn't kept, but it was probably done to keep in continuity with the other Sonic titles where he can't go Super with the Chaos Emeralds alone, instead needing a stronger power source (The Super Emeralds) to achieve his form. It has also been alluded to that Yuji Naka said that the reason he can't go Super with the normal emeralds is due to "inexperience" by his young age, and thus why he has a Super form with the Super Emeralds.