The thermal subsystem maintains the right temperatures in all parts of the spacecraft. That may sound easy, but it turns out that it's not. The Sun heats up one side of the spacecraft, and black space on the other side pulls the heat out. The hot side is thus hundreds of degrees hotter than the cold side. In addition, parts of the spacecraft that use electrical power will generate heat internally and tend to get very hot. Now, on Earth, when your home gets too hot you can cool things off with fans or air conditioners. Or, when it gets too cold, you turn on the furnace. All these methods work by adding or subtracting heat from air and then moving it around (this is called convection). In space there isn't any air, so convection doesn't work. Other physical processes -- conduction and radiation -- must be used to move heat around the spacecraft and ultimately get rid of the excess.
Conduction is the process by which heat moves through substances and between substances that are in contact, like pool water and your skin, or a cold metal railing and your hand. Heat moves around the spacecraft mainly by conduction (to a lesser extent also by radiation).
Radiation, or more correctly, electromagnetic radiation, includes everything from X-rays to sunlight to radio waves. Our eyes are sensitive to only a very small part of this spectrum, the part we call visible light. The heat you feel on your face when you get close to a fire is the result of infrared radiation, which is a type of electromagnetic radiation just beyond the range your eyes can see. The only way a spacecraft can actually absorb or get rid of heat is by electromagnetic radiation.
Mars Reconnaissance Orbiter employs several conduction- and radiation-based techniques for thermal control: