Many spiritual practices describe a kind of subtle energy, sometimes called qi or prana, that is considered to be the very essence of our life force. In yoga, this life energy runs through countless pathways called nadis (some texts estimate there are 72,000 nadis throughout the body channeling prana to each and every cell).
Along the nadis there are energy centers called chakras. Think of the nadis like lots of little rivers and streams, and the chakras like pools and reservoirs, or perhaps more accurately, whirlpools. Chakra is a sanskrit word translated as "wheel", so the chakras are essentially wheels of energy.