Sound and gameplay mechanics were altered radically from the first two entries into the Pac-Man series - instead of eating dots, the player is required to eat keys in order to open doors, which open up sections of the maze that contain what in earlier games were known as "fruits" (foods such as apples and bananas, or other prizes such as Galaxian flagships), which are now the basic items that must be cleared. Once all the food is eaten, the player advances to the next level, in which the food is worth more points. In earlier levels, keys unlock nearby doors, while as the player progresses through the levels, it is more common for keys to open faraway doors.
In addition to the original power pellet which allows Pac-Man to eat the ghosts, a "Super" pellet was introduced which turns Pac-Man into Super Pac-Man—in this form, he is twice as large, moves much faster and has the ability to eat his way through barriers without unlocking them. In this state, he is invulnerable to the ghosts. The ghosts appear flat and thin in order to give the illusion of Super Pac-Man "flying" over them. He still cannot eat them without the help of the original power-up. This game also adds a speed button, which allows Pac-Man to move faster when in Super mode. Pac-man can enter the ghost house in this state, too, able to eat hiding ghosts.
Source: Wikipedia, "Super Pac-Man", available under the CC-BY-SA License.