The basic effect of heat transfer is that the particles of one substance collide with the particles of another substance. The more energetic substance will typically lose internal energy (i.e. "cool down") while the less energetic substance will gain internal energy (i.e. "heat up").

The most blatant effect of this in our day-to-day life is a phase transition, where a substance changes from one state of matter to another, such as ice melting from a solid to a liquid as it absorbs heat. The water contains more internal energy (i.e. the water molecules are moving around faster) than in the ice.

In addition, many substances go through either thermal expansion or thermal contraction as they gain and lose internal energy. Water (and other liquids) often expands as it freezes, which anyone who has put a drink with a cap in the freezer for too long has discovered.