There are 5 basic stages of critical hits + stage 7 which assures sure critical hit (only frostbreath and storm throw have this effect though)
Super Luck increases the chances of having a critical hit by double stage, which means moves like Slash increase by double percentage, Slash was a move that has stage 2 critical hit ratio and not stage 1 which is 6.25% so the percentage would be 33.3% or 1/3 if you have a pokemon with super luck ability.
Some items also increases the chances of critical hits such as the following items:
Lansat Berry - A Poffin ingredient. If held by a Pokémon, it raises its critical-hit ratio in a pinch.
Razor Claw - Raises chances of landing a Critical Hit. Evolves Sneasel into Weavile at Night.
Scope Lens - Increases chances of landing a Critical Hit
Lucky Punch - If attached to Chansey, Critical Hit ratio rises
Stick - If attached to Farfetch'd, Critical Hit ratio rises