The Little has the ability to shrink anything including one's magical power. Its main body is just like the one depicted on its card: a tiny child with blonde hair in a yellow jester's outfit. Little's true form has also been shown to be very nimble and able to jump to great heights for it's size (not like The Jump, though). It is under The Light and is the sister card to The Big.
The Little first appears in Episode 24. Little had shrunk Sakura to the card spirit's size and her magic, so she could not use any Clow Cards. Sakura chased Little all around the first floor of her house, stopping The Little from shrinking Touya as well. Then Little jumped up the stairs to Sakura's room, but she could follow as she could not use any of the Clow Cards. Cerberus then helps Sakura get to her room where she literally runs into Little and reverts to her normal size, finally sealing the card.
The Little was transformed in Episode 55 along with The Big. Sakura was trapped in the story of Alice in Wonderland, where the characters were replaced with people she knew, due to another spell by Eriol. She had tried to use both Big and Little to her advantage, but it didn't work.