"The Chakras are thought to vitalize the physical body and to be associated with interactions of a physical, emotional and mental nature. They are considered loci of life energy, or prana, (also called shakti, or chi), which is thought to flow among them along pathways called nadis. The function of the Chakras is to spin and draw in this Universal Life Force Energy to keep the spiritual, mental, emotional and physical health of the body in balance." -Wikipedia, and I'd say it's a decent definition.
My definition is simply that Chakras are energy filters located in the body with corresponding locations in your ethereal or astral body that act as conduits or connectors between you and the Universal Life Energy. Each Chakra filters the Universal Life Energy into a different type of energy(emotional, mental, physical, spiritual, etc) that your body requires to function properly.
In a nutshell, Chakras are energy centers within the body.
There are seven major Chakras, but really there are many more, including some that reside outside of our physical and etheric bodies. For now, we'll just talk about the major seven. Here is a list of their location, color, and function starting at the bottom and working our way up.
Name: Root Chakra.
Color: Red.
Location: The base of the pelvic bone.
Function: This Chakra represents the physical, root, survival, security, your base.
Name: Sacral Chakra.
Color: Orange.
Location: The groin area, lower abdomen.
Function: Emotions, sexual energy, creativity.
Name: Solar Plexus.
Color: Yellow.
Location: The stomach or gut.
Function: Personal power, control, freedom, mental energy.
Name: Heart Chakra.
Color: Green.
Location: Heart and lung area.
Function: Love, compassion, empathy, healing.
Name: Throat Chakra.
Color: Blue.
Location: Throat.
Function: Communication, expression, speech.
Name: Third Eye.
Color: Indigo/purple.
Location: Right between your eyebrows.
Function: Intuition, psychic abilities, clairvoyance.
Name: Crown Chakra.
Color: White or Violet.
Location: Top of your head.
Function: Consciousness, connection to the Universal Life Energy.
Your Chakras are always open, for the most part, but they may not be in optimal shape or as open as they could be. There are many ways to open up your Chakras, as with cleaning them you must find the method that is right for you.
Beginning with the root Chakra imagine a red ball of energy spinning in a clockwise direction around an imaginary pole going through your body from head to toe. Make the red ball of energy strong, clear, and powerful. Sometimes people let their Chakras pulsate with energy, sometimes Chakras look like spinning galaxies(that's how I visualize them), and sometimes they are cylindrical instead of in the shape of a ball. All you're really doing is activating the energy center. It may feel like you're imagining it at first, but don't let that stop you. In time you'll feel, see, or even hear a tonal sound when you spin your Chakras. Repeat the process with your second Chakra (orange) and all the way up to your Crown Chakra (white or violet).
As for closing down your Chakras, you don't really want to do that. The more energized your Chakras are, the better you will feel, and the better your life will flow. What you may be thinking of is shielding yourself, which is something you want to do if you are around someone with aggressive energy, or someone trying to leech your power from you. To shield you can visualize a shield of white or gold energy surrounding your whole body, leaving you cocoon like inside. Or you can visualize a mirror in front of you reflecting the person's negative energy back on them.
As mentioned in my definition of Chakra I consider them to be like filters. Have you ever seen a coffee filter or vacuum cleaner's filter after it's been used? They get really gunky and filthy, they can't be used a second time. Here are a few examples of if your Chakras are clogged.
Clogged Root Chakra: You may feel paranoid or as if you are in danger when you are safe.
Clogged Sacral Chakra: Your emotions may be offset, you may overreact to minor things.
Clogged Solar Plexus Chakra: You probably feel scared or victimized.
Clogged Heart Chakra: You probably feel very disconnected from other people.
Clogged Throat Chakra: You may feel like you're being suppressed or you've got something you want to say that you're not saying.
Clogged Third Eye Chakra: Your intuition is probably not getting through, your instincts may be slightly off.
Clogged Crown Chakra: You probably feel disconnected from the Universal Life Energy.
Clogged Chakras can cause physical problems related to the area where they function. For example, if someone is intimidating you, you might have digestive issues in your Solar Plexus (i.e. nervous stomach). If there is someone you really need to talk to but you just can't bring yourself to do it, you may end up with a sore throat if your Throat Chakra is clogged. Clogged Third Eye could result in a headache. And so on.
If you are empathic or psychic, you may be attracting other people's emotions and issues to you like a magnet. Other people's issues might stick to you because you don't release them, and that can cause your Chakras to get a little clogged as well. There is no earthly reason to carry around someone else's burdens or pain. You're just gumming up your own system.
Some people unconsciously (or even consciously) close their Chakras because they are afraid of the connection they are feeling to themselves, other people, and to the world around them. Naturally, this isn't a good thing to do and if you close your Chakras you'll need(in my opinion) to open them back up and face your fears.
In the physical world filters need to be replaced after being used to much, but fortunately we don't have to remove our Chakra, buy a new one, and install it. We can clean these filters out.
Everyone has a different best/favorite method. Some people just do as their Chakra is supposed to do, forcing the Chakra to open up and flow. For example if your Throat Chakra is clogged; write a speech, find an audience, and give it to them verbally. If your Third Eye is closed, rely on your intuition so that the Third Eye is forced open again. Other people simply meditate and focus on balancing themselves out, using visualization to clean and clear the clogged Chakras.
Forcing the Chakras to clear and open up is easy, I've already posted lessons for meditation, but for visualization it's best to know what to visualize. Locate your Root Chakra and imagine a red energy spinning clockwise in that location. Use the force of the spin to remove and clear any dark spots or psychic gunk that has accumulated there. Just imagine it all spinning away. When that Chakra is spinning with clear, red energy, you're done. Move your awareness to the next Chakra, the Sacral Chakra, and do the same thing. Spin it clockwise, imagining an orange light whirling around in the area of the Sacral Chakra. Clear it and move to the next Chakra. Do this all the way to your Crown Chakra.
Some people may cleanse one, stop visualizing it, move on to the next, cleanse, stop visualizing that one, move on to the next, and so on, but I like to keep visualizing each one even though I may switch to focusing on the next one(as needed) while cleansing. At the end some people like to pour white light all around their body, and sometimes they add golden light for protection, or green light for healing, etc. How you end is up to you, the important thing is that your Chakras are cleared and cleansed.
There are many ways to clear or cleanse your Chakras and in doing this you may open them up more as well. Find the method that works for you and if you don't see it in this thread feel free to share your method here.
Cleansing your Chakras should only take a few minutes. If you do this every day, you'll notice less and less psychic gunk and you can get them clear really fast (within 30 seconds or so).
When your Root Chakra is closed you may not feel safe. You may feel paranoid, in danger, alarmed. Obviously, this is not a good way to live. Nobody should have to live in paranoia for any reason. Having your Root Chakra open will not only help you to feel safe, but also help you to realize when you really are in danger, help you to be alert, to defend yourself or flee accordingly, having your Root Chakra open can keep you alive.
A closed Sacral Chakra can result in mood swings, depression, unexplained anger, and a lack of self control. Opening the Sacral Chakra will balance this out, opening it up all the way will result in complete control of your emotions. You will know fear only when you accept it, feel anger only when you wish for it, and have peace for as long as you desire it.
If your Solar Plexus is closed you may feel trapped or victimized. You may feel powerless, out of control, and exhausted. Opening up your Solar Plexus gives you the sense of freedom, power, strength, you will have more energy and you will have better self control. By opening your Solar Plexus you go from being the victim to being the victor.
If your Heart Chakra is closed you may feel isolated from other people. You may feel unloved, uncaring, and hateful. Opening your Heart Chakra not only allows you to feel loved by others, including yourself, it also makes it easier for you to care and not feel hate toward others. Besides that it will also help you to heal from the emotional injuries caused by others.
When your Throat Chakra is closed you may find it hard to say what needs to be said, to express yourself and your beliefs, and to communicate with others. Opening your Throat Chakra makes it easier for you to express yourself, to share your beliefs, to communicate with friends and family. It also allows you to be more diplomatic and charismatic.
Your Third Eye Chakra gives you intuition, psychic abilities, clairvoyance, etc. With it closed you would be a 'normal' human being since most humans close the third eye. With it open, however, you can gain many abilities. The ability to read minds, to gaze into the future, see Spirits, Demons, and Angels. Things that most people call 'paranormal.'
The Crown Chakra is your consciousness. Without it, you may find it harder to decipher the difference between what is right and what is wrong and to feel connected to the world around you. With it open you'll find it easier to reap the benefits of every other Chakra, you'll find it easier to feel good about what you do, and you'll feel a little more at peace no matter where you go.
Play with your Chakras; you won't go blind. It can take time to become aware of them and feel the energy. Don't give up. Learning to keep your Chakras healthy and powerful is important to overall spiritual health.