Heat is a form of energy that flows from warmer objects to cooler objects.
Most people use the word heat to describe something that feels warm, however in science, thermodynamic equations, in particular, heat is defined as the flow of energy between two systems by means of kinetic energy. This can take the form of transferring energy from a warm object to a cooler object. More simply put, heat energy, also called thermal energy or simply heat, is transferred from one location to another by particles bouncing into each other. All matter contains heat energy, and the more heat energy that is present, the hotter an item or area will be.
Heat vs. Temperature
The distinction between heat and temperature is subtle but very important. Heat refers to the transfer of energy between systems (or bodies), whereas temperature is determined by the energy contained within a singular system (or body). In other words, heat is energy, while temperature is a measure of energy. Adding heat will increase a body's temperature while removing heat will lower the temperature, thus changes in temperature are the result of the presence of heat, or conversely, the lack of heat.
You can measure the temperature of a room by placing a thermometer in the room and measuring the ambient air temperature. You can add heat to a room by turning on a space heater. As the heat is added to the room, the temperature rises.
Particles have more energy at higher temperatures, and as this energy is transferred from one system to another, the fast-moving particles will collide with slower moving particles. As they collide, the faster particle will transfer some of its energy to the slower particle, and the process will continue until all the particles are operating at the same rate. This is called thermal equilibrium.
Units of Heat
The SI unit for heat is a form of energy called the joule (J). Heat is frequently also measured in the calorie (cal), which is defined as "the amount of heat required to raise the temperature of one gram of water from 14.5 degrees Celsius to 15.5 degrees Celsius." Heat is also sometimes measured in "British thermal units" or Btu.