In the Hunter x Hunter universe there are Professional Hunters - people who are hired as mercenaries to fulfill tasks, which can be as simple as delivering a letter, to dispatching a local gang. Professional Hunters are required to know Nen.
Nen is the technique to control your life energy, in short terms. The ability to use Nen is strongly linked to the emotions and mental state of it's user, and as such has a large versality by many users. No two Nen users will be exactly the same even if their Aura Type is.
Aura is the life energy produced by all living bodies that is vital for survival. It is continuously produced from within the body without human interference and in people without the ability to use Nen it is insubstantial. Typically aura is invisible to people unless they can use Gyo. A person will have a Hatsu (Aura) Type from the time they're born. Aura is usually clear colored and surrounds the body closely unless using a technique. Some people can have different colored auras, such as purple, gold, blue, etc. which is determined by the type of Hatsu type they are as well as the abilities they employ.
To learn Nen a student must train to open and close their aura nodes (the points at which aura comes from within the body). Typically it would be done gradually, through meditation. However, in circumstances where one must learn Nen quickly they can go through Baptism, or Intiation. This method is dangerous and has the risks of permanent disability or death.
Some people are more adapt at learning Nen and are called geniuses, psychics or superhumans by the general public. These people will naturally learn how to use Nen without training and excel in their fields of practice. Generally, they don't know that they are capable of Nen unless somebody approaches and tells them first.
FOUR MAJOR PRINCIPLES OF NEN USAGE
Ten - Zetsu - Ren - Hatsu
Ten (Envelope; Shroud)
When one opens their aura nodes they must learn to keep their aura from leaking away from their body. The process involves keeping the nodes open, but having the aura flow through and around the body. It creates a shroud around the user, protecting them from emotional attacks. The other effect of Ten is reducing the speed at which your body ages and begins to break down.
Zetsu (Supress; Null)
Zetsu is the principle of stopping the flow of aura altogether by closing all of one's aura nodes. This is useful for going unnoticed by other Nen users, and makes the user less visible on the Nen radar. Zetsu, however, leaves the person using it wide open for Nen attacks, which can deal lethal damage unless you have your aura to protect you.
Ren (Refine; Enhance)
It is the direct application of Ten, causing the user to produce a high amount of aura around themselves. While Ten is defensive, you could consider Ren offensive. It increases the user's physical strength and durability, as well as their aura pool that they can draw on for advanced techniques.
Hatsu (Release; Act)
It is the transmission of one's aura so it can be projected to carry out a certain function. Hatsu is the user's personal expression of Nen, and it creates a special ability which is called a Nen Ability. It is used to determine which Hatsu Type a person is.
Gyo - In - En - Shu - Ken - Ko - Ryu
After learning the Four Major Principles and practicing upon them, one can begin to use the advanced techniques provided their affluence with Nen is strong enough.
An advanced application of Ren in which the user concentrates a larger amount of aura into one specific body part. Gyo increases the strength of that one body part, but leaves the rest of the body more vulnerable. Gyo is used in the eyes to see aura and things hidden by In. Using this allows one to see what kind of attacks another Nen user is using against them.
An advanced form of Zetsu, used to almost completely conceal the aura of it's user. In can be used on the user's Nen ability to make it invisible to other Nen users. As previously stated above, a person can see through In with Gyo, but also by En.
An advanced application of Ren and Ten. In Ren aura usually envelops only a small amount of space around the user's body. En is when one extends their Ren so that their aura extends further than normal, and during this they can use Ten to shape their aura (sphere is a typical shape). This technique allows them to sense the shape and movement of anything entering their aura. The use of maintaining a large En aura is very tiring on the user. Training at En can make the radius of one's aura larger, with auras as large as 50m spherical radius. There are always exceptions to the rule, with extremely powerful personages being able to extend it to 300m.
An advanced application of Ten, Shu allows a Nen user to extend their aura's envelopment onto an object, giving the object the same protection or strength as the user would give their body.
An advanced version of Ren. Ken allows a Nen user to maintain a state of Ren over their entire body. This allows the user to defend themselves from all directions without needing to use Gyo. It's tiring to maintain and takes training to help maintain it for longer periods. While it is a useful technique, it is not as strong as when Gyo is applied to a specific part of the body.
Is an advanced version of Gyo in which all of the user's aura is concentrated into a particular body part. Zetsu is used completely stop the flow of Nen in all other parts of the body. This makes the body part being enhanced by Ko to become exceptionally powerful. Doing this will leave the rest of the body unprotected from Nen attacks.
Ryu is the term for real time use of Gyo (the adjustment of aura concentrations in various body parts) by a Nen user in combat. When you use Gyo to increase the power in your fist as you punch with it, or when you strengthen a leg to withstand an attack, it is called Ryo.
ENHANCER - TRANSMUTER - CONJURER - EMITTER - MANIPULATOR - SPECIALIST
Your primary Aura Type is the one you can use at 100% efficiency, while the ones immediately adjacent are 80% efficiency. The Aura Type opposite to your primary type is 40% efficiency while the other ones are 60%. So, in short, if you're an Enhancer you're: 100% ENH, 80% EMIT - TRANS, 60% MANIP - CONJ. The reason Specialization wasn't included in the previous list was because you can't partially use Specialization, you either can or can not. That being said, Conjurers and Manipulators have 1% efficiency potential for Specialization due to being adjacent to it. (And thus higher chances of being Specialists)
Use their aura to power up their own natural abilities or that of an object, increasing their physical attack and defense. Enhancers are best suited for melee combat. That being said, Enhancement is the most balanced of all types and allows it's users to spread themselves evenly between offense and defense. They are generally good at using simple abilities, however more complex Enhancer abilities include increasing one's own healing.
Can change the properties of their aura to mimic something else, such as electricity. The things a Transmuter chooses their aura to mimic don't necessarily have to be based on real things, and they can have it mimic two things at once (rubber and gum, electricity and fire, etc.).
It is the ability to create a physical, independent object out of the Conjurer's aura. They can conjure and dispell it their own disgression, and don't need to be necessarily next to the thing they've conjured. They can leave it miles away and it will still continue to exist. However, to make the object they've conjured stronger they must set a rule, or condition, to the object. (This vaccum can suck up things infinitely as long as it isn't a living thing, etc.)
Their abilities are very similar to Conjuration in that they can create and deploy their aura, separating it from their body. Aura decreases in intensity very quickly as it leaves the body usually, but if an Emitter is strong enough they can maintain the separated aura for long periods of time. They must control the aura they've deployed using Manipulation skills, whereas Conjurers don't need to.
Their abilities allow them to control living or non-living things in certain ways, and the degree of control is limited by the condition they put on the ability. The bigger the risk to themselves, the more powerful the ability becomes. The condition can be something such as "attach an object to the enemy, touch the enemy on the back".
Specialization is anything not belonging to the other categories and is the vaguest of the Aura Types. It is usually controlled by conditions set upon the abilities the user chooses, just like Manipulation, Conjuration, and Transmission. With conditions set that involve the risks on the user's part they can employ powerful abilities such as "being able to use all Aura Types at 100% efficiency against a certain enemy at the loss of my life if I use it against anyone else". It is possible to become a Specialist later in life if you're not born into the Aura Type specifically, but it is very rare.
AURA PERSONALITY STEREOTYPES
Enhancers are typically determined, simple and honest. Their words and actions are often dominated by their feelings, and they are usually selfish and focused on only their goals.
Transmuters are whimsical liars. They have unique personalities and are regarded usually as weirdos, or cheaters. They are secretive, rarely revealing their true intentions.
Conjurers are cautious and high strung. Their caution makes them very observant and they rarely fall for traps or lies. Calm analysis is the strength of the Conjurer.
Emitters are impatient and not fussed with the details of a situation. Quick tempered and hot blooded are the typical personality traits of an Emitter and they share the impulsivity of an Enhancer. Emitters, however, forget things more easily and are able to calm down faster.
Manipulators are logical, going into everything at their own pace. They are argumentative and protective of their loved ones. They are focused on their goals, and do not want to hear what other people might have to say about them.
Specialists are individualistic, often the center of attention whether they mean to or not - their charisma draws people in. They will regale people of their triumphs, but they tend to be stand-offish about becoming close friends with other people.
Nen responds to the goals, strengths, and desires of it's individual users. A student can increase an individual skill by placing a self-imposed restriction (Vow) upon it. The Vows can be anything, such as "I will only use my power on people with brown eyes, I will lose my hair if I don't". The higher the punishment one sets to their Vow should they break it increases the strength of the ability in question exponentially. Vows that have the user's death as the punishment are extremely strong but only follow that Vow. Typically, setting Vows is seen as a liability considering the risks.
USER CREATED TECHNIQUES
In the anime (and manga) the characters begin to learn specific techniques that are very individualistic. They will follow your Hatsu Type in the way that they fundamentally function (Enhancers would have more body based techniques, where as Conjurers can have a sword that can fly next to them and function on it's own). Considering the Hatsu Type, the Four Major Principles, and the Advanced Techniques when creating a character is important and can dramatically change the way you want your character to perform and function.
PROFESSIONAL HUNTER TYPES
Archaeological Hunters - Beast Hunters - Blacklist Hunters - Contract Hunters - Crime Hunters - Gourmet Hunters - Hacker Hunters - Information Hunters - Jackpot Hunters - Lost Hunters - Music Hunters - Sea Hunters - Temp Hunters - Treasure Hunters - Virus Hunters - Youth and Beauty Hunters