"We are all cells in the same body of humanity."
Peace Pilgrim
The Reiki system as Mikao Usui developed knows three
degrees or levels through which you gradually open up for a great
amount of high vibrational energy. All other Reiki systems are
based on the original Usui Reiki Ryoho.
Reiki I
Usui called the first degree in Reiki Shoden, or
First Teaching. This is the practitioners level in which you learn
to heal yourself and physically present persons.
After having received the initiation(s) (traditionally 4, non-traditionally
1) all you have to do is to lay your hands on someone or something
with the intent to heal and the energy will flow through you.
The Reiki symbols are placed in your aura and chakras but they
are not revealed to you.
Reiki II
This second level is called Okuden, or Inner Teaching,
in Japanese.
The most important thing in Okuden is learning to do absent or
distance healings. You are taught how to heal people or animals
not present, and how to send energy to a goal or situation.
You will receive another attunement which allows you to further
open up your energy system, and it is believed that the amount
of energy channeled with Reiki II is twice as much as the energy
of Reiki I.
Three symbols and their mantras are now given, so you can consciously
learn to make use of them.

Japanese draught-screen 16th century
Reiki III
Shinpiden, or Mystery Teaching, is the third and
highest level in Reiki. It is the full empowerment.
You will receive a final attunement and learn to work with the
fourth symbol, the master symbol. The amount of energy you channel
with Reiki III becomes much stronger.
You'll become able to teach and initiate others in Reiki, and
that's why third degree Reiki is usually called master/teacher
level.
Other Reiki branches may add more levels to their
system, but these extra levels do not supply extra energy, it
simply means that there are more sub-levels.
Originally it took two or more years to become a
fully empowered Reiki channel, but in these times of greatly accelerated
energy we need less time in between the several degrees.
Nowadays there are even singular attunements which raise your
vibration and energetic openness immediately to the level of master/teacher.