Every atom is physically bonded to its neighbours in some way. If heat energy is supplied to one part of a solid, the atoms vibrate faster.

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Conduction in Non-Metals
Every atom is physically bonded to its neighbours
in some way. If heat energy is supplied to one
part of a solid, the atoms vibrate faster. As
they vibrate more, the bonds between atoms are
shaken more. This passes vibrations on to the
next atom, and so on                          
          Eventually the energy spreads
throughout the solid. The overall temperature has
increased.
2
Conduction in Metals
All metals are good conductors of electricity.
For a similar reason, they are also good
conductors of heat.                            
        In metals, not only do the atoms
vibrate more when heated, but the free electrons
charge around more as well. These transfer the
energy much faster than just vibrations in bonds.
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Conduction Summary
Conduction happens mainly in solids.
          All atoms vibrate, but vibrate more
when heated.           Heat spreads by
conduction when atoms increase their vibrations,
and pass this energy on to those nearby.
          In metals, free electrons carry the
heat energy faster than the atomic vibrations...
          ...and transfer it by colliding
with other electrons and atoms.          
Because of this, metals are the best
conductors of heat energy.
4
Convection
Perhaps the first thing that most people say is
"heat rises". Whilst not wrong, what you should
say is "hot air rises" or "hot water rises".
Anything fluid - that is gases or liquids -
will tend to change density with changes in
temperature. For example, if heated, air
decreases in density. The surrounding air is
cooler and denser. This makes it heavier, so it
falls beneath the hot air, forcing it upwards.
5
Convection in rooms
In cooler countries, many houses have radiators
to heat their rooms. This is a bad name for them
- as they give off heat mainly by convection!
                                        The
convector heater warms the air in contact with
it. This becomes less dense, and rises. The
ceiling forces this air to circulate as shown,
warming the air around it. Finally, when the air
has cooled, it falls downwards, completing the
cycle.
6
Convection Everywhere
Convection currents occur wherever fluids are
being heated. Here are some more examples Water
in kettles Soup in saucepans Water in a hot
water tank Convection even occurs in the sun!
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Convection Summary
Convection occurs in gases and liquids.
          Hot fluids rise, cold fluids fall.
          Convection currents occur because heat
is lost from the rising fluid, cooling it down.
          The whole fluid will rise in
temperature as a result of mixing caused by
convection currents.
8
Radiation
As you may have read elsewhere, heat energy can
be transferred by radiation. This is infra red
waves, a type of light.                    
Just like light, infra red waves can travel in
a vacuum. This is why we can feel the sun's
warmth - this heat has travelled 92 million miles
to reach us!
9
Radiation
Heat radiation has the useful property of being
able to travel in all directions - unlike in
convection. Heating in bathrooms is often
provided by radiant heaters, which safely reflect
heat from a bar that is hot due to its electrical
resistance                                    
                          The orange glow of
the bar is visible light. Remember most of the
radiation coming off it is infra red... heat. But
like light, it travels in straight lines.
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Radiation Summary
Heat radiation consists of infra red waves.
          These travel at the speed of light, in
straight lines.           Heat radiation can
go in any direction, even through a vacuum.
          Heat reaches us from the Sun by IR
radiation.