Shelter protects your body from the outside elements. This includes heat, cold, rain, snow, the sun, and wind. It also protects you from insects and other creatures that seek to do you harm.
The survival expert has several layers of shelter to think about. The first layer of shelter is the clothing you choose to wear. Your clothing is of vital importance and must be wisely chosen according to the environment you are likely to find yourself in. Be sure to dress in layers in order to maximize your ability to adapt to changing conditions. The next layer of shelter is the one you may have to build yourself, a lean-to or debris hut perhaps. This is only limited by your inventiveness and ingenuity. If the situation requires, your shelter can be insulated with whatever is at hand for the purpose. Being prepared, you may have a space blanket or tarp with you, in which case creating a shelter should be relatively easy. Before you are in need of making a survival shelter, be sure to practice and experiment with a variety of materials and survival scenarios on a regular basis. Should the need arise you will be glad you did.