Putting the garden to bed for the winter is mostly a matter of cleaning up and covering up. As winter progresses and temperatures drop, those plants that aren't killed outright by frost prepare for dormancy. Clear out the blackened stems and foliage of annual flowers and vegetables to prevent the possibility of their harboring disease pathogens and insect eggs over the winter. The cool weather is a good time to make a cold frame, dig and box in raised beds, and make general repairs.
While it appears as if all activity in the garden has stopped, there's a lot going on under the soil until it freezes. Newly transplanted trees and shrubs, divisions of perennials, and hardy bulbs are all growing roots, drawing on soil nutrients and moisture around them. Earthworms and various microbes in the soil are still processing the organic material they're finding. Most likely, the organic mulch you spread to protect the soil during the summer months has substantially decomposed. It's important to spread new mulch now -- a thicker winter layer -- to protect plants and soil over the winter months. The idea is not so much to keep the soil warm as it is to keep the temperature even. Once the soil is frozen, mulch keeps it frozen. So if you have shade trees, convert the fallen leaves to mulch and use it throughout your property.