Heat transmission takes place in three ways: conduction, convection and radiation. Conduction is the transfer of heat between two bodies that are in direct contact. For example, when you touch a hot pan it feels hot as it is good conductor of heat.
Convection is the way that heat is transferred in liquids and gases. Hot liquids and gases rise up and transfer the heat to the surrounding molecules of the liquid or gas. This makes them cooler again and they fall down.
Radiation is the transfer of heat by electromagnetic waves. Hot bodies emit electromagnetic waves which carry energy. This energy is transferred to the objects on which the radiation falls. This is how the heat from the Sun reaches the Earth.