Reiki is an energetic healing practice developed in 1922 by Dr.Mikao Usui. The etymology of the word reiki comes from the Japanese word rei (霊) which means spirit, or universal life, and ki (気) meaning energy. The history of reiki can be all over the place at times, and can have many different points of view depending upon who you are talking to, though it is important to know that the three big forefathers of this healing art were Dr.Mikao Usui, Chujiro Hayashi, and Madam Takata. They all played a major part in the formation of reiki, along with popularizing it in the western world.
Practitioners of reiki primarily, become attuned to reiki through other reiki practitioners that have gone through the attunement process. The attunement process is done to connect other individuals with the energy source that is reiki allowing them to be able to harness more of the reiki energy in different ways. This is usually done through a three-step process of attunement usually labeled as levels 1, 2, and 3, but can change depending upon tradition. Once a practitioner becomes attuned with the power of reiki they are able to work with it in order to bring healing to themselves, and others. All three reiki attunements can be done at once, or they can be spread out, but this will depend on tradition, or student and teacher. Once you have been attuned to reiki energy you never have to be re-attuned, though some people go through the attunement process again to reinforce the connection, or as a way of getting initiated into a different tradition of reiki healing.
Nontraditionally some practitioners have discovered ways to self attune themselves to the energy of reiki, allowing themselves to practice without going through an attunement with a reiki master that is part of the traditional energetic lineage of Dr.Mikao Usui.
The levels of attunement:
The first level of attunement allows you to begin working with reiki energy.
The second level of attunement allows you to begin working with the first three reiki symbols. These symbols are Cho Ku Rei, Sei He Ki, and Hon Sha Ze Sho Nen.
The third level of Attunement allows you to begin working with the final Master symbol known as Dai Ko Myo, and also in some traditions gives you the title of reiki master.