Chakras are energy centers or energy organs within each person. Every person has hundreds of chakras. All living beings have chakras, including the earth. Note that there are many different chakra systems by different peoples. Modern derivative chakra systems usually outline 7 chakras.
Here are two chakra definitions found in "The Subtle Body - An Encyclopedia of Your Energetic Anatomy" by Cyndi Dale:
1. Based on analyses of the word chakra: "A chakra is a circular-shaped energy body that directs life energy for physical and spiritual well-being."
2. Synthesized definition of chakras from a scientific point of view: "Chakras are energy transformers, capable of shifting energy from a higher to a lower vibration and vice versa."
The chakras can be visualized as a pair of conical vortices emanating from the front and the back of the body, centered in the spine. Usually the 7 chakras are placed along the spine as follows:
First chakra At the bottom of the spine
Second chakra Abdomen
Third chakra Solar Plexus
Fourth chakra Heart
Fifth chakra Throat
Sixth chakra Forehead (middle) - Third eye
Seventh chakra In the top of the head
Generally, the function of the chakras is to transfer energy from the subtle body to the physical body. So chakras take energy in, 'translate it' for the physical body to be able to receive, and then send it to the physical body. The translated energy is distributed in the physical body through the nadis (prana or life force energy conductors), that in turn are connected to the physical nervous system. Each chakra is usually associated with an endocrine gland.
Nearly every cultural chakra system considers the chakras a vital part of an enlightenment or spiritualization process. Chakras don't only vitalize our physical body, they also bring about the development of self consciousness and transmit spiritual energy in order to bring the individual into a state of spiritual being.