There are three basic forms of heat transfer that can cause your home to lose heat in the winter or gain heat in the summer.

In conduction the heat energy is transferred without the movement of any substance. The heat energy moves along or across an object or substance. For example, when you place an iron pan on the range or oven and heat it up, you know that the handle can get hot. The heat energy from the oven burner is transferred to the bottom of the pan and through the pan to the handle by conduction.
The rapid motion of the molecules at the hot end of the object and collisions with the colder slower moving molecules to the right, cause these molecules to move faster and increase their temperature. This process continues, with the temperature of the object increasing from the left side to the right side. This transfer of heat energy from the left to the right through the object is called conduction.

In convection the transfer of heat energy is caused by the flow of a fluid, most commonly air or water. The hotter fluid tends to be less dense and therefore rise, causing what are called convection currents. The hot air will rise in one part of the room, forcing the cooler air above to fall in another part of the room. The net result is a transfer of heat energy, again from the hotter fluid to the colder fluid. This is how the heating system in your home transfers the energy into the house. Whether the heating is done by hot water that is circulating in pipes or hot air that is circulating in ducts, the temperature in each room is increased (or decreased) by the convection currents.

The transfer of heat or energy that requires neither an object or a flow of a fluid is radiation. In radiation heat transfer the energy is moved by the changing of heat energy into light energy or light energy into heat energy. A burning fire is one example. Certainly, the fire will heat the air and cause heat transfer by convection. But much of the chemical energy released by the burning of the wood or whatever fuel is being used will be changed into light energy. When this light energy, probably in the infrared frequency, hits you or surrounding objects, it is changed into heat energy.