There are three different types of Haki that can be used to sense people's spiritual energy and predict their actions, give the user a protective coating of spiritual energy, and, for a certain group of "chosen ones", overpower the willpower of others.
There are three major types of Haki: Observational, Armament, and Conqueror's. Observational Haki was the first type of Haki to be introduced in the series. It appeared in the Skypiea Arc, well before the Marineford Arc, though it was called Mantra by the citizens of the Cloud Island.
Monkey D. Luffy has mastered all three types of haki, including Kenbunshoku Haki (Observation Haki), Busoshoku Haki (Armament Haki), and Haoshoku Haki (Conqueror's Haki). He can even use several subforms of the hakis, which makes him one of the most powerful haki users in the One Piece world.
Those who have mastered the fourth form of Haki will be able to manipulate nature as they see fit. They can cause living organisms to grow rapidly or die rapidly (though not resurrect them), and make objects move with the force of their minds, akin to telekinesis.
Roger used advanced forms of conqueror's haki as he imbued his sword with conqueror's haki and clashed with Whitebeard. He also used advanced armament haki for more powerful sword attacks. Additionally, Roger possessed the observation haki, thus, making him the strongest haki user of all time in One Piece.
Conqueror's Haki is one of the rarest powers to exist in the One Piece world. Unlike the other two Haki forms, this power isn't accessible by everyone but exists within only a chosen few. Its basic level allows the user to intimidate and overwhelm those who have weaker wills.
Despite its great capabilities, the Haki in a person's body is not limitless and it can be depleted from overuse, rendering the user unable to use it for a set period of time while it regenerates.
Roronoa Zoro is a member of the Straw Hat Pirates. Zoro was the first person who accepted Luffy's invitation and joined the latter on his journey to become the Pirate King. Zoro is a very powerful fighter and he possesses all three types of Haki.
As fans know, Nami doesn't wield any form of Haki in the story so far. Haki is a power that Rayleigh described to be the manifestation of the user's spirit. Every person in the One Piece world possesses Haki, however, not everyone is able to bring this latent skill out.
Sanji is an extremely skilled Haki user. During the two years he spent at Maiden Island, he trained his usage of both Armament and Observation Haki. Unlike Zoro and Luffy, Sanji wasn't taught this skill by anyone. He managed to learn all by himself and that is certainly an incredible feat.
Roger. Conqueror's Haki, also known as the Haki of the Supreme King, is the rarest form of Haki that only a few people in the world of One Piece are born with. Unlike the other two Haki types, Conqueror's Haki can't be learned through training and is only possessed by those who are born with the spirit of a king.
4 Armament Haki Subtype 5: Tribal Haki
Although much about this Armament Haki subtype (colloquially named Tribal Haki by fans) remains unknown, it is speculated that it allows users to combine their Haki with their Devil Fruit powers.
Three overarching forms of Haki (Armament, Observation, and Conqueror's) exist in One Piece, each of which is divided into further subclasses.
While this is shown to be a very rare gift among the world as a whole, it is noted by Don Chinjao that the New World is filled with numerous others who possess the ability to use this Haki. He explains that Luffy will most certainly encounter them and fight them for the title of Pirate King.
The circumstances surrounding Zoro's left eye aren't a complete mystery; at the very least fans know that it happened sometime during the two-year time skip. During this time, each member of the Straw Hat crew committed to training and growing stronger.
In the anime, when Luffy first found Nidai Kitetsu, he actually detected it with Kenbunshoku Haki. This ability can also be utilized to measure the Haki of others.
By training with the people of Momoiro Island, Sanji was able to tap into the skill of Armament Haki at some point during the timeskip.
A glimpse of Shanks' power reveals that he holds the title of "The Killer of Observation Haki," and here's why. Shanks is one of the strongest characters in Eiichiro Oda's One Piece.
Weakest Observation Haki users are Aisa and Otohime. Both were born with the ability to use Colour of Observation, but since they are not fighters, they don't have any capabilities in combat.
The Armament haki (busoshoku) is the ability to project your spirit into armor. An example of how it represents one's spirit is Vergo's purple haki as per debut. Vergo maybe mastered his armament haki. So this purple tint to his haki could've been his spirit's nature, the same to Luffy where he got his own shade.
In the anime, the user of Haoshoku Haki has a dark purple aura around him.
White Haki spirit color is the rarest color and cannot be obtained with robux.