Conduction is the movement of heat through a substance by the collision of molecules. Two objects must be touching for this transfer to occur. On the boundary of a hot object the faster-moving molecules collide with the slower-moving molecules on the boundary of the cooler object. In this way, the slower-moving molecules gain energy and move faster. They strike other molecule inside the cooler object. Heat energy is spread throughout the object. Conduction is a good way to spread heat energy in solids.
In liquids and gases, convection is usually more efficient. Convection is the spreading of heat energy by movements of heated liquids or gases, such as water and air. To understand how this works, picture a fireplace. As the fire heats the air around it, the warm air expands and takes up more space. The warm air is less dense and floats above the more dense cool air. The warm air rises up the chimney, taking the smoke and ashes with it. Cool air from the room flows into the fire from all sides to take the place of the rising warm air. A continuous circulation pattern of hot and cold air is set up in the room. This example of convection demonstrates how heat energy is moved around by the circulation of air.
In that sense, a heat pipe is a solid conductor and heat moves through it. In that scenario it could be both conduction and convection. However, convection is usually more associated with free air.
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