So God made Adam and Eve and told them they could have everything except for the fruit on one tree but they got greedy and ate it and then they achieved knowledge they weren't supposed to so God punished them...wait wrong text. So this rabbit chick ate a piece of fruit off a sacred tree which gave her a power known as chakra. After eating this, however, that tree turned into a horrific beast that caused unspeakable death and destruction. Huh you know this sounds very familiar!
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In all seriousness, chakra is the energy the flows in, now every living entity as a result of the Rabbit Goddess who was the first to bare chakra by eating the forbidden fruit off the sacred life tree (or whatever it was called). It became the source of life for all living beings as well as the power source for shinobi. It's akin to Qi in the Chinese if we relate it back to real terms. What we, in the RP call raw chakra consists of two parts: spiritual and physical or Yin-Yang. The spiritual governs the imagination and concepts the jutsu while the physical governs reality and manifests the concepts from the spiritual to reality making the otherwise immpossible a reality. That is what we call ninjutsu. There are other forms of chakra, the nature chakra and natural chakra/senjutsu chakra. The former consisting of the following five elements; fire, water, earth, lightning, wind which possess specific properties that allow one to manipulate said elements. Advancing further there are advanced elements that are the result of combining two or more of these elements such as Lava (fire, earth) or Ice (water, wind). These are usually the result of possessing a kekkei genkai or Kekkei Tota which allows a person to manipulate two elements simultaneously where as ordinary people can only do one at a time (bar advanced users in YY in our RP but that's moot). Moving onto Senjutsu chakra. Senjutsu chakra is the combination of natural energy/chakra that is taken externally from the nature into one's body and harmoniously joined together with one's internal Yin-Yang chakra in equal thirds. This is different from Orochimaru's brand of curse seals which does exhibit the use of natural energy however it is not manipulated evenly with one's chakra thus the reason for the deformities that come as a result of the imbalance. There is also other forms of chakra such as star energy in the non-canon Hoshigakure, Bijuu chakra which is chakra derrived from the 9 bijuu and the Juubi, and other demons from the movies that aren't exactly canon. However this is all chakra. The absence of chakra is the absence of energy which is essentially death.
Chakra is controlled through the tenkensu points (or chakra points) of the body which resemble the meridians in the Chinese if we talk acupuncture. They're located all over the body and a shinobi learns to manipulate their chakra into these points in order to project their chakra to create/execute their techniques. As is in the name 'chakra control' the users controls the type and flow of chakra to a degree that allows them to execute techniques of various sizes, shapes, density, power, precision, etc. There are two fundamental types of manipulation; shape and nature. Shape manipulation deals with the shape chakra takes (examples being fireball jutsu, rasengan, rasenshuriken, etc). Nature manipulation is the manipulation of the elements as mentioned above. Accessing the natures deep inside the user and projecting them outwards. These two types of manipulation are often enough used in conjunction with one another such as the aforementioned rasenshuriken and fireball jutsu. In short, the manipulation of chakra is what allows nininjutsu, genjutsu, and to a degree some taijutsu, to exist.
Fuinjutsu is a specific subset of ninjutsu that deals with containing, pretty much anything. Fuin meaning sealing techniques seals, binds, restricts, contents to a specific area. Techniques like the generic sealing technique as an example we can use it to seal weapons into scrolls. When the jutsu is used on a weapon it is then confined to that scroll until accessed again. Some sealing techniques aren't two ways (sealing the entity and then reverseing/unsealing). Orochimaru's brand of curse seal deals with sealing natural energy into another and when the seal is unlocked by the party who was branded, the chakra is accessed. Bijuu are confined to seals inside of hosts known as Jinchuuriki but can also be sealed in other objects as we've seen the Hachibi sealed by Sandaime Raikage in an item until re-sealed into Kirabi. Barriers, as I understand, were recently added as a sub-category of fuinjutsu as the barrier too deal with containment of a certain area just like fuinjutsu. In a barrier, an entity is confined to a specific area or protecting a certian area (depending on it's function). I understand this as I disagreed with it before. However this is everything I have to start out with about the basics of everything.
To touch briefly on the basics of ninja as I've already started. Ninja became a thing with the creation of Ninjutsu, which became a thing with the invention of Ninshu, which became a thing when Hogomo discovered it, which was the result of his mother eating shit she had no business in eating. This is not a slippery slope fallacy, that is exactly what happened. After the life tree became the Juubi the Rabbit Goddess had two sons who fought it and Hogomo was the only person to get credit for it when the other mofo was probably the one saving his ass I bet (grimy). However he was the one who discovered Ninshu which was the ideal of using chakra to unify people in a state of equality. However the natural greed for power, desire to be individual and achieve more broke that chain and resulted in secular interests in chakra causing people to develop their own prowess in the arts of chakra. These arts became known as ninjutsu and associated with war, while the practitioners of chakra became known as shinobi (or at least most did)