Reiki, simply put, is a method of accessing the Ki (called, in other cultures, Chi or Mana) which is all-encompassing in the universe, and channeling it to strengthen and heal nature, animals, people, and situations.
Although anyone can do Reiki, with or without attunement, many people are not usually aware of the Ki, or Universal Life Energy because is that it carries a vibration higher than most people currently can touch with their spirits. Just as there are notes and frequencies above the pitch that the human ear can hear, so too there are spiritual and energy vibrations above the 'pitch' that incarnate spirits can feel.
However, Reiki provides a method to attune individuals to 'hear' this 'pitch', in a more precise form, to make it easier for everyone can access the Ki and use it to heal both themselves and the Universe. A Reiki master has been attuned to the level necessary to attune others to the Ki.
An individual who is using Reiki does not send the energy; rather, he or she opens a channel through which the Ki can flow. The one receiving the Ki will automatically accept the Ki as needed.
Reiki is not a religion, although persons of a religious nature often incorporate Reiki with their personal religious practices.