Haki s a mysterious force that is found in every living being in the world. It is not that different from the normal senses. However, most people do not notice it or fail to awaken it. Broadly, there are two types of Haki common to everyone, given the proper training; however, there is a third type that only a certain group of "chosen ones" are said to possess
Haki is dormant in every living person, but it's rare for most people to ever awaken that ability. Intense training can awaken it, as can extreme shock, such as with Coby during the battle of Marineford.
There are three types of Haki in the world
Kenbunshoku Haki, also known as Mantra in Skypiea, is a form of Haki that allows the user to sense the presence of others, even if they are concealed from view or too far to see naturally. When using this type of Haki, one can predict an opponent's moves shortly before they make them, thereby making the attack that much easier to evade. This prediction appears to the user as an image or brief "premonition" of what the opponent will do in the user's mind's eye, and the damage the user will take if the attack actually "hits". It appears that the more killing intent the enemy has, the easier they are to predict. Although, more skilled users can predict future moves whether there are ambient murderous intents or not.
Busoshoku Haki allows the user to create a force similar to an invisible armor around themselves Similar to Tekkai, this allows the user to defend against attacks that would otherwise cause them harm. Only significant physical force can overcome this defense much like Luffy using Gear Second. The invisible armor can take on many forms such as Luffy hardening his fist to the point where it changed color. With training, it can be further honed into a weapon with which to strike others. It can be used to augment the user's own attacks, projected ahead of the user as pure brute force, or imbued in weapons to increase their effectiveness. Any attack enhanced by Busoshoku Haki hits immensely harder than it would normally without it.
Haoshoku Haki is a rare form of Haki that cannot be attained through training and only one in a million people carry it. This type of Haki grants the user the ability to overpower the wills of others. The most common usage in the series so far is knocking those with weak wills unconscious. Whilst inexperienced users are restricted to merely overpowering the will of one individual or blindly knocking out weak-willed people around them, those with more expertise can pick out weak-willed individuals in a large group and knock them out without affecting those around them However, those with stronger wills can resist or even ignore the effects of Haoshoku Haki. According to Rayleigh, while this type of Haki cannot be attained through training, it can be improved through training, as it grows as the users spirit grows. It seems that upon recovery, victims of Haoshoku Haki experience the sensation of chills running through their body. However, it presumably wears off after a short period.