Each of the 7 levels of Reiki correspond to one of the 7 main chakras or energy points on the body.  It is an ascension process through the chakras, highlighting each energy centre and balancing in the heart - raising the vibration of the individual at each level. This is done via Attunement's at each level (a specific formula of symbols over the head and hands). The higher the vibration rate of the body the faster the body regenerates and recovers from illness.

Information is available on all 7-practitioner levels. 
Each level corresponds to a main energy centre in the body

Reiki 1 - highlights the base and crown
Reiki 2 - highlights the sacral centre, 3 symbols and many tools learnt
Reiki 3 - highlights the solar plexus, the first out of four attunements are taught that are used to align someone permanently to Reiki, and a booster attunement for Reiki 2, 1 symbol learnt
Reiki 4 - highlights the heart, 1 symbol learnt
Reiki 5 -highlights the throat, booster attunements for Reiki 3 & 4 learnt, 1 symbol learnt.
Reiki 6 - highlights the third eye, energy work (symbol learnt in Reiki 2)
Reiki 7 - highlights the crown, booster attunement for Reiki 5,6,7, and crown symbol learnt.

So then, all of the practitioner levels create a strong, balanced, clear energy. For example, level 5 is fun to watch a shift in people as it opens the throat...some quiet people walk in, do Reiki 5, then walk out booming their truth and vocalising themselves clearly! Each level opens and balances at each level.

At this point I would just like to say, that it is unnecessary for everyone to do every level. Just Reiki 1 is huge and beneficial to give Reiki to self and others. Reiki 1 is a strong energy, bringing balance and freedom through the hands. Anything beyond Reiki 1 is a choice of how each person feels.

At each level there is a change in energy to the amount of Reiki that comes through the practitioner.  Each level increases the vibration of the individual and ability to access greater amounts of this pure life force energy. It is a complete system through each energy centre with all levels balancing into the heart while maintaining a strong grounding energy.  Reiki is a complete, precise system for self-transformation and expansion of consciousness.

The benefits of Reiki are infinite and everyone can benefit from learning Reiki for themselves.  The advantage of learning for yourself is to be able to permanently boost your life force energy that you walk with day to day and have an even greater supply to access for bringing greater health to yourself and others – true self empowerment.