Critical attacks usually occur by chance, and are attacks that do higher damage than a normal attack.What level you are and what weapon you use can also change what effect a critical hit has. You cannot choose to use a critical attack because they just happen by chance. For example, you level up high enough with claws that you have a higher chance of delivering a critical hit with that weapon, it means that not only will the damage be higher, but it will also be more often. There is also an axe skill (Hatchet man)  which fails often, but delivers a critical attack when it works.