Reiatsu

is the physical force/pressure that a person’s Reiryoku creates when released. Most Shinigami and Arrancar, even Quincy and Bounts can manipulate the release of their reiatsu.

Reiatsu VS Reiryoku
The difference between spiritual pressure and spiritual energy is simple: Reiryoku is the amount of energy a being has stored within their body or soul, whereas Reiatsu is the pressure that a person’s Reiryoku exerts. The difference is much like energy and power. Power being the amount of energy exerted per unit of time. Reiryoku is potential while Reiatsu is energy in use. In general, those with high levels of Reiryoku will often have the highest levels of Reiatsu also. A skilled warrior can overcome a person possessing greater Spirit Energy by possessing greater Spiritual Pressure.

Reiatsu Colors
Different spirits have different colors of reiatsu, and it does not depend on the character's race or power; instead, the color is determined by the owner’s personality.

Shape
Some particular characters have a shape or manifestation of their energy whenever they release enough of it. The manifestations are one of the characters inner strength/energy.

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 Hollows and Shinigami have this type, while every Quincy and Humans always have this type. Fullbringers are the only Humans shown to have Hollow-like powers. Yasutora Sado noticed 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 Visored and Arrancar have this specific reiatsu. For Visored, they feel more like Shinigami, while Arrancar feel more like Hollows. When Ichigo's inner Hollow started to take over Orihime could sense it, becoming afraid of how heavy and dark it was. The Visored, strangely enough, also feel more like Hollows 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
Three characters have been shown using elemental reiatsu, even without releasing their Zanpakutō, or it being their natural Spiritual Power:

Fire & Heat: Genryūsai Shigekuni Yamamoto. Captain 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.

Ice & Cold: Rukia Kuchiki & 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.