Reiatsu

is the physical force/pressure that a person’s Reiryoku creates when released. Most Shinigami and Arrancar, even Quincies and Bount can manipulate the release of their reiatsu. 1st Division Captain Shigekuni Yamamoto-Genryūsai used his spiritual pressure in the form of a stare to paralyze Nanao Ise. Yachiru Kusajishi, the Lieutenant of the 11th Division can emit her spiritual pressure into the form of a pink cat. The most powerful Shinigami such as Ichigo Kurosaki, Kenpachi Zaraki, Sōsuke Aizen, Commander-General Yamamoto, Byakuya Kuchiki, Rukia Kuchiki, Renji Abarai and others have vast amounts of Spiritual Energy, which they can emit in the form of Spiritual Pressure with devastating effects.

Reiatsu VS Reiryoku
The difference between spiritual pressure and spiritual energy is simple: Reiryoku is the energy reserves a spirit being has stored within their body or soul. Reiatsu is this same energy released into the air, causing dramatic effects. For example, Ichigo has a vast amount of reiryoku, so much so that it's like a faucet that's never turned off. This causes his prescence to be felt by pretty much anyone who has spiritual awareness. However, during his fight with Byakuya he releases this energy into the air, creating a huge amount of reiatsu; It was strong enough to be felt through the entire area of Seireitei.

Colors
Different Shinigami have different colors of reiatsu. It does not depend on the race or power of the character. The color depends completely on the owner’s personality. For instance, Grimmjow's reiatsu is blue, Ulquiorra's is green and Nnoitra's is golden. Ichigo's starts out blue in Shikai and when using his resolve, as is Tōshirō Hitsugaya's and Uryū Ishida's; Kenpachi's reiatsu is dark yellow, Renji's is red, Yachiru's is pink and Yumichika's is green. Albeit, Quincy tend to have a blue variation of reiatsu considering it is one of their signature colors along with white.

Another thing worth noting is how reiatsu color can change. In Byakuya's fight with Ichigo Kurosaki his spiritual energy was blue, even in Bankai as it increased. Now ever since his first fight with Ulquiorra it is always black with a red outline while using Tenza Zangetsu, seemingly leaking out of his body. When Renji and Captain Kuchiki are ordered to apprehend Rukia, the attacks from his blade leave a bright green energy on impact in its sealed state. Since then it has become red.

Shape
Some particular characters have a shape or manifestation of their energy whenever they release enough of it, similar to a Zanpakutō spirit, though completely different. This "shape" has been demonstrated by three characters so far, all in the same battle. As Ichigo and Kenpachi fight each other with all their might in one final clash of power, they reveal the "shape" of their reiatsu. Ichigo's takes the form of his hollow mask made up of blue energy, while Zaraki's takes the form of a Giant Skull made of yellow energy. As Yachiru Kusajishi watches the pair fight, a subordinate attempts to give her an urgent message. She scares him away when he persists, showing a cat shape created from her own pink energy. Kenpachi's Skull has been shown once again in his fight against the 5th Espada, showing the full release of his power. All of these manifestations are not related to Zanpakutō releases whatsoever, but are more like the wielder's own inner strength/energy. Kenpachi does not know the name of his Zanpakutō, while Kurosaki's Zangetsu is a constant release type and Yachiru has yet to reveal her shikai.

Defensive use
With sufficient enough power, Shinigami can use their own spiritual pressure to protect themselves from attacks, their pressure surrounding their skin. Kenpachi Zaraki was the first to demonstrate this use of reiatsu. When fighting Ichigo Kurosaki, he opened up the chest area of his Shihakushō, and ordered Ichigo to cut him. When attacking with all his strength, Ichigo found Kenpachi uninjured, while his own hands were bleeding. It was not until he increased his own reiatsu that he was able to cut Zaraki. Later on when the Arrancar appear, hierro is introduced. One particular Espada, Nnoitra Jiruga, had the most powerful hierro of any Espada. Kurosaki Ichigo was unable to pierce his skin, as was Yasutora Sado. Kenpachi himself had great difficulty injuring him, until he adapted his reiatsu, finally cutting, and eventually killing him.

Psychological use
Reiatsu at its most basic level can instill fear in others when it is very high (in comparison to the one feeling it.) This has been shown when Ichigo fought Kenpachi, and also when Nanao Ise assisted against Yamamoto. Both Nanao and Kurosaki felt it was impossible to win, making their own reiatsu far weaker than normal. Kurosaki was able to fight Zaraki only once he was able to counter with his reiatsu. Willpower is the simplest way to fight this method.

Offensive use
In the Arrancar Arc, both Renji and Hitsugaya released their limiters, unleashing a huge amount of spiritual pressure. Both Shawlong Qufang and Yylfordt Granz were slightly frozen and burned respectively. During the fight with Szayel Aporro Granz, Renji Abarai once again used spiritual pressure transferred through his blade in order to injure the Espada. His reiatsu was red and it was able to cut his face since he was holding it Zabimaru so close. This use only occurs when the user is releasing a large amount of energy at once or in an emotional state.

Paralysis
If the reiatsu is powerful enough, Shinigami can paralyze an opponent just by focusing pressure through their stare. During the Soul Society Arc when confronted the Commander-General, Captains Jūshirō Ukitake and Shunsui Kyōraku fled the scene to find a safer battlefield. Lieutenant Nanao Ise followed them and prepared to assist her captain. When contemplating whether or not they could defeat Yamamoto, he stares directly into her eyes, effectively paralyzing her and causing enough fear to make her believe he was impossible to defeat. Aizen and Gin have also been shown to be able to use this ability.

Standard type
Most characters have this form of reiatsu, which have no other effect than those described above, along with its unique color and feeling. Most Shinigami and Hollow have this type, while every Quincy and Human always have this type. Yasutora Sado is the only human shown to have hollow-like powers, so naturally he felt more at home in Hueco Mundo, and his reiatsu feels like a hollow despite his species; It is still a standard type.

Dual type
All Arrancar and Vizard have this specific reiatsu. For Vizard they feel more like Shinigami, while Arrancar feel more like hollow. When Ichigo's inner hollow started to take over Orihime could sense it, becoming afraid of how heavy and dark it was. The Arrancar, strangely enough, also feel more like hollow when they release their powers. Arrancar seal their hollow abilities, meaning they become as close to their original form as can be, explaining this phenomenon.

Element Type
Two characters have been shown using elemental reiatsu, even without releasing their Zanpakutō: General Yamamoto and Tōshirō Hitsugaya. When Tōshirō was a child, he had trouble controlling his power. In his sleep it would go out of control, affecting his grandma and causing her to shrink thanks to the sheer cold. This cold power caused others to fear him. Yamamoto has used his burning spiritual pressure to heat up his sealed Zanpakutō, causing it to glow dark orange like a recently forged sword. He has also displayed a large amount of flames emanating from his body before his shikai, which is needed for his fire based techniques.