Chakras are considered to be centres of energy throughout the body - seven main ones in alignment from the base of the spine upwards - which can be activated to enhance every aspect of one's mental and physical state.
These same centres of energy are used in harmony or individually by "Healers," as part of the "balancing," "harmonizing," and "Healing" process, and their vibrations radiate specific colours which form the basis of "Colour Therapy."
Whilst there is no clinical evidence for their existence, they've been an intrinsic part of eastern philosophy and understanding for centuries, and are very much at the heart of modern day Yoga and Meditation practices.
It's impossible to be concise when talking about Chakras and their usage, and as a Meditation teacher and Healer I cover the subject in great detail with all of my students. Following is a breakdown from my website http://www.meditation-for-all.com which should help with your understanding.
Chakra One: Red (Root Chakra - Muladhara)
Located at the base of the spine, this Chakra forms our foundation. It represents the element earth, and is therefore related to our survival instincts, and to our sense of grounding and connection to our bodies and the physical plane. Ideally this Chakra brings us health, security, and dynamic presence.
The Root Chakra is about being physically there and feeling at home in situations. It grounds us in the physical world. When open you feel stable and secure, and don't unnecessarily distrust people. You feel present in the here and now and connected to your physical body. You feel you have sufficient territory. When under-active, you tend to be fearful or feel nervous and unwelcome. When over-active, you may be very materialistic and greedy, obsessed with being secure and resistant to change.
Chakra Two: Orange (Sacral Chakra - Swadhisthana)
Located in the abdomen beneath the Navel, Spleen, lower back, and sexual organs. Related to emotions and sexuality. It connects us to others through feeling, desire, sensation, and movement. Ideally this Chakra brings us fluidity and grace, depth of feeling, sexual fulfillment, and the ability to accept change.
When open your feelings flow freely without being over emotional. You’re open to intimacy and you can be passionate and lively. You feel in control and you have sufficient self esteem. When under-active, you tend to be passive, timid, indecisive, unemotional, and “poker faced.” When over-active, you tend to be emotional all of the time, emotionally attached to people, very sexual, domineering and probably even aggressive. Blockage manifests as emotional problems or sexual guilt.
Chakra Three: Yellow (Naval Chakra - Manipura)
Located in the solar plexus and known as the “Power” Chakra. It rules our personal power, will, and autonomy, as well as our metabolism. When healthy, this Chakra brings us energy, effectiveness, spontaneity, and non-dominating power. It is our seat of emotions and gives us a sense of personal power in the world.
When open you feel in control and you have sufficient self esteem. When under-active, you tend to be passive, timid, and indecisive. When over-active, you are domineering and probably even aggressive. Blockage manifests as anger or a sense of victimization.
Chakra Four: Green (Heart Chakra - Anahata)
This Chakra is called the Heart Chakra and is the middle Chakra in a system of seven. It is related to love and is the integrator of opposites in the psyche, mind and body, male and female, persona and shadow, ego and unity. A healthy fourth Chakra allows us to love deeply, feel compassion, and enjoy a deep sense of balance and peace. The Heart Chakra is about love, kindness and affection.
When open, you are compassionate and friendly, and you work at harmonious relationships. When under-active you’re cold and distant. When over-active you’re suffocating people with your love which probably has quite selfish reasons. Blockage can manifest as immune system or heart problems.
Chakra Five: Blue (Throat Chakra - Vishuddha)
Located in the throat and related to communication and creativity. Here we experience the world symbolically through vibration, such as the vibration of sound representing language. The Throat Chakra is about self-expression and talking. When open you have no problems expressing yourself.
When under-active you tend to be quiet, and can be introverted and shy. When over-active you’re a bad listener and tend to speak too much - usually to domineer and keep people at a distance.
Chakra Six: Indigo (Brow/Third Eye Chakra - Ajna)
This Chakra is known as the Brow Chakra or “Third Eye Center” or “Adjna Center.” Often referred to as the Pineal Gland. It is related to the act of seeing intuitively. As such it opens our psychic faculties and our understanding of archetypal levels. When healthy it allows us to see clearly, in effect, letting us "see the big picture." The Third Eye Chakra is about insight and visualisation.
When open you have good intuition. When under-active, you're not very good at thinking for yourself, get confused easily, and rely too much on beliefs. When over-active, you tend to live in a world of fantasy and may experience hallucinations.
Chakra Seven: Violet (Crown Chakra - Sahasrara)
This is the Crown Chakra that relates to consciousness as pure awareness. It's our connection to the greater world beyond, to a timeless, spaceless place of all-knowing. When developed, this Chakra brings us knowledge, wisdom, understanding, spiritual connection, and bliss.
The Crown Chakra is about wisdom and being one with the world. When open you're unprejudiced and quite aware of the world and yourself. When under-active your spiritual awareness will be low and you're likely to be rigid in your thinking. When over-active, you’re probably intellectualizing things too much, may be addicted to spirituality, and are probably ignoring your bodily needs.