Shadow Form

Shadow Form is one of Sora's forms from Kingdom Hearts.

Temporary Form
Sora turned into a Shadow Heartless after his second fight with Riku. He is only in this form briefly and can only jump and run around. (If the Start button is pressed when Sora is in this form, Sora's menu cannot be accessed.) However, due to Kairi's heart, he was able to recover into his human form again.

Side Effects
Since Sora became a Heartless, albeit for a short time, he created a Nobody that became known as Roxas. Oddly, Sora could not perform a lunge attack like all other Shadows. This probably has something to do with Sora's strong heart, which is the reason he can wield the Keyblade. Heartless only attack beings of light, or Nobodies, but because there were only Heartless around, Sora couldn't attack anything. Kairi's being there can be explained by Sora's strong heart, meaning he can't attack beings of light either, meaning he can't attack at all.

While this event gave birth to Roxas, it also gave birth to the unusual Nobody, Naminé, who is born from Sora's body and soul, but from Kairi's heart. That is the very reason Naminé has control over Sora's memory in Kingdom Hearts Chain of Memories.

It is confirmed by Tetsuya Nomura that this event triggered a side effect and created the Anti Form when Sora was given his new clothes and Drive Form abilities a year later.

Confusion
Shadow Form is often confused with two other forms of Sora in Kingdom Hearts.

Shadow Form- Is the form Sora took when he gave his heart to free Kairi and became a Heartless in Hollow Bastion.

Anti-Sora- Is a Heartless created by Riku to demonstrate to the power Maleficent had given him in Neverland.

Anti Form- Is a Drive Form that is the darkness in Sora from when he was in Shadow Form.

Trivia

 * During this form, Sora is unable to activate any of the lift nodes to bring him back to the ground floor, and has to rely on jumping off the sides of the building. However, if he jumps too far and lands in the ravine below, the lift node activates to bring himself back to ground level.
 * Sora stated to Kairi that while a Heartless he nearly forgot everything.
 * The reason Sora became a normal shadow is because his heart doesn't have that much darkness in it.