The research of thermal energy started in the 4th century BC, by Aristotle. James Joule was the first person to receive credit for his research on loss and gains of heat. He is not the first person to use the term thermal energy. Thomas Young was the first person to use the term energy. Thermal energy is energy that comes from a heat source. It's only  one of many types of energy, but it's a kinetic energy that uses motion. Kinetic energy is required in order to produce heat. This energy can be used to produce work.

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Heat are small particles within an object. The faster these particle move, the more heat is produced. Particles must be in motion to transfer heat from one another. The energy that comes off this heat is measured in Joules (J). To convert energy from heat a device such as an engine is needed. Otherwise it is hard to convert. The transfer of heat can be made three ways: Convection, Conduction, and Radiation. They commonly come from fossil fuels, such as coal and gas.

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"Matter is made up of particles or molecules. These molecules move (or vibrate) constantly. A rise in the temperature of matter makes the particles vibrate faster. Thermal energy is what we call energy that comes from the temperature of matter. The hotter the substance, the more its molecules vibrate, and therefore the higher its thermal energy."

http://www.eschooltoday.com/energy/kinds-of-energy/what-is-thermal-energy.html

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"It is easy to confuse thermal energy and heat, or even think that the terms can be used interchangeably. However, the two terms are quite different. Thermal energy is the energy that comes from heat. When thermal energy is transferred, it is called heat. Heat is a temperature and is measured in degrees of Fahrenheit or Celsius. An object can contain thermal energy, but it cannot contain heat."

https://www.reference.com/science/thermal-energy-used-cd6635831221da64

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