Each spell is graded on a scale from 1 to 99, the latter being the most powerful and the most difficult to perform. The scale is a measure of difficulty, as there can be multiple spells which share the same level. To use Kidō, a Shinigami must recite the specific incantation for the spell, which is often long and requires a few seconds to speak.
This weakness can be negated by Shinigami capable of using Eishohaki, a technique that releases Kidō without a spirit chant. While it decreases the time needed to release the Kidō, it drastically weakens the spell. While anyone well versed in Kidō can use this technique, it is most effective when done by a skilled practitioner, otherwise the spell could utterly fail. The most skilled experts are capable of using this technique to amazing effect.
The power of a spell relies on the power of the user, as even a low-level spell can be utterly devastating when utilized by a high-class Shinigami.
There are three main types of Kidō: binding spells, destruction spells, and healing spells.
Bakudō: are a broad category of defensive spells which block or repel attacks or freezes enemies in place.
Hadō: refer to offensive spells which harm an enemy or cause damage to things.
Healing spells: do not have known names, numbers, or incantations to cast, and simply heal the target. As the user holds his/her hands above the patients wounds, his/her palms glow with green spiritual energy to facilitate healing. For the mechanics of healing, the patient's Reiatsu is replenished first. Then the body's Reiatsu works with the healer's Reiatsu to heal the patient's body.
Though Kidō is commonly used by Shinigami in combat, it has several weaknesses that make it difficult to use for that purpose. All of the spells have fairly long incantations and require concentration; because of this, the person using the Kidō leaves themself open to attack during the duration of the incantation. In addition, if an opponent is familiar with Kidō, there is the possibility of them identifying the spell by its incantation before it can even be used. However, highly skilled Kidō users can use various techniques to counter these weaknesses, such as Eishohaki and "Double Incantation", which involves mixing the incantations of two or more Kidō spells together to save time and confuse ones opponent. A rarer use of Kidō is mixing various spells (such as Bakudō and Hadō together) for unique effects, which is the mark of skilled user. Kidō is commonly seen used in instances of surprise attacks rather then all out assaults, being most effective when one's opponent doesn't see it coming. Kidō is also usable with just the spell name and no incantation or spell number.
[Taken from Bleach Wiki]