Heat is a measure of energy.
When a substance is heated, it gains energy.
Energy makes gas or liquid particles move around faster, and solid particles vibrate more rapidly.
Heat energy is measured on an absolute scale (meaning it can't go lower than zero).
The unit of heat energy is the joule (J).
Temperature is a measure of hotness.
Temperature is usually measured in degrees Celsius.
Energy normally flows from hot objects to cooler one, eg. from someone with warm hands to someone with cold hands.
If there's a difference in temperature between two objects, energy will flow between them.