Reversals
Known as Reversals in most other Fighting Games, a Wakeup Attack is performed when your character gets up off the ground. If you time a Special Attack the instant you get up, you'll perform a Wakeup Attack. However, not all Special Attacks can be used as a Wakeup Attack. For example, Scorpion can only use the Teleport and Takedown as Wakeup Attacks.
The benefit of performing a Wakeup Attack is that your move gains 8 frames of invulnerability. That allows many moves that you would think cannot be used as a Wakeup Attack to beat out a lot of attacks. For example, Ermac can easily use his Force Push as a Wakeup Attack. Though it has no invulnerability on its own, the 8 frames of additional invincibility from being used as a Wakeup Attack will make it beat out just about every attack, as it hits before the 8 frames of invulnerability run out.
You can actually perform Wakeup Attacks after a Roll Recovery as well. However, if you perform a Delayed Wakeup, you can no longer trigger Wakeup Attacks. Also, you can perform either a Special Attack or an Enhanced Special Attack as a Wakeup Attack.