Chakras are the energy centers of the body that have physical, energetic, emotional, intellectual and spiritual components. Although there are hundreds and hundreds of chakras throughout the body, there are 7 primary ones in the physical body and 2 just beyond the body but are just as important in my opinion (Earth Star and Soul Star). 
Chakra is a Sanskrit word for “wheel.” While the chakra system developed within Hinduism, there is a corresponding system within the Christian Sacraments of the Catholic Church and the Tree of Life within the Jewish Kabbalah
Even though they are different systems, I find the chakra system of the body to be very complimentary with Reiki. They both are about becoming your Authentic Self, getting to know who you really are at your essence. Reiki accomplishes this by expanding your awareness to a higher plane of consciousness where there is peace and unconditional love, through sensing the wholeness of being. The chakra system works from a different perspective, in that breaking down the different components of the whole.