Kraid is one of the main bosses Samus encounters in Metroid (and its subsequent remake, Metroid: Zero Mission) and Super Metroid. In the original game, Kraid was part of the miniboss duo along with Ridley, implying he is ranked fairly highly in the Space Pirate forces. In Super Metroid, he is one of the four bosses you must defeat in order to open the way to the Space Pirate base, Tourian, in order to face Mother Brain.
In Super Metroid, Kraid is an enormous reptilian creature, taking up roughly two screens in height. His main attacks were spikes that shot out of his belly that could be used as platforms, and throwing talons that could be destroyed for item pickups. Due to the limitations of the NES, Kraid in the original Metroid was much smaller, but shared the characteristics of firing spikes from his belly. In the remake of Metroid, Metroid: Zero Mission, Kraid resembles his Super Nintendo form, implying that this is his 'real' intended form and not the diminutive one in the original. In the NES game, doing enough damage to Kraid with any weapons eventually destroyed him. In Zero Mission and Super Metroid, Samus had to jump on the spikes that shot from Kraid's belly in order to fire missiles into his open mouth, his only weak point.
In both the original Metroid and Super Metroid, there was a 'fake' Kraid that was smaller and much weaker than the larger boss Kraid. They were able to be dispatched rather easily by Samus, and respawned when she re-entered the room. Curiously, the 'fake' Kraid is missing in Metroid: Zero Mission.
Kraid also appears in Super Smash Bros: Melee as an unlockable Trophy, as well as a cameo in the stage 'Brinstar Depths'. He periodically rose from the lava behind the stage, slamming it and causing the stage to rotate.
Kraid was originally intended to be a boss in Metroid Prime, although due to time constraints he was removed from the production. His appearance is similar to his SNES and GBA counterparts, although without a snout and a metal dome covering his head.