What is heat?   Heat is a form of energy that can be transferred between a system and its surroundings because of a temperature difference between them.

How is heat transferred?   The transfer of energy by heat flow is caused by molecular collisions; this type of interaction is termed a thermal interaction.

When does heat transfer stop?   When a system reaches thermal equilibrium with its surroundings---the system and surroundings have the same temperature.

For two systems at the same temperature and in thermal contact, a steady-state temperature difference cannot be generated.