Chakra (C̣akra, meaning "wheel, circle"), sometimes spelled Cakra or Cakka, is an energy center of subtle body believed to be psychic-energy centers in the esoteric traditions of Indian religions (Buddhism, Hinduism, Jain, etc...). There is no consensus on the number of chakras we have, their location, descriptions, or even the purpose or function of them. Some say there are 5 chakras, other traditions say there are 12, minor and majors. They are believed to be along or inside the spine, in a special meridian (or Nadi called Sushumna).
Buddhist yogis developed one of the earliest models of the chakras, 1 500 years ago. They helped develop the Tantra school of yoga. Their map showed five chakras, one for each of the meditation Buddhas. In the Tantra school of yoga, as practices in India by Hindu yogis, seven major plexuses were detected, one for each heavenly plane of existence, ranging from the earth to the highest heaven.
Chakra is not another term for the nerve plexus or endocrine glands of the physical body, although they may reside in the same general location. Similarly, chakras are not physical organs of the body. When trying to describe where a chakra "is", one finds oneself in a dilemma. Common language suggest chakras are physically located in the spine, but we can bear in mind that this is true and false. Paul Grilley gives this example, which I believe to be meaningful : " A broken heart is a real experience that indeed seems centered in the heart, bur that is not where the feelings actually resides. The chakras have have a physical correspondence, but they are more than physical. Don't be limited by only physical conceptions"
It is safe to say that chakra is a center of subtle energy that needs to be manipulated with care in order to achieve complete physical and spiritual health, and eventual enlightenment. The practice of yoga helps to awaken the chakras, allowing prana to flow though them.