Your sleeping system consists of all the equipment you use to keep yourself warm, excluding your shelter. A complete system will include a sleeping bag, sleeping bag liner, sleeping pad, and a ground cloth.
The core component of any sleeping system is a sleeping bag. This your primary source of insulation, they can be anything from an emergency bivvy to a high-end mummy bag.
Sleeping Bag Liner
A sleeping bag liner is placed between you and your bag, it is generally thin and made of an easily washable material. Liners serve two purposes, to add a few degrees of warm to the bag and to keep the bag clean. When you use a liner you keep any dirt, oil, or other contaminants from touch the bag itself so that you don't have to clean it very often.
A sleeping pad insulates you from the ground, which can absorb heat from your body at a considerable rate. Your body weight may compress the insulation in the bottom of your bag, rendering is less effective. There is some debate over the necessity of a sleeping pad.
A ground cloth is a small tarp that protects your sleeping pad from water, dirt, and wear & tear.