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Kinetic energy (KE) is the energy an object has inherent in its motion. For example, an object coasting along a sheet of ice could be tied via a pulley to a weight and lift the weight, i.e., the energy of the object's motion used to do some work. Another clear instance is the action of a roller coaster: at the top of one of its hills, the train has the potential energy associated with its mass located at a height above ground. This potential energy is converted to kinetic energy as the train descends. The kinetic energy of the train at the bottom powers the train's climb up the next hill. Classical kinetic energy formula:
1
ke = ——— mv²
2
Relativistic:
mc²
ke = ————————————— — mc²
√ 1 - v²/c²
The relativistic formula produces virtually the same amount as the classical formula unless the speed is very fast, i.e., a good fraction of the speed of light. Thermal energy (aka heat energy), the energy due to a substance's temperature, is the collective kinetic energy of all the particles' individual movements. Sometimes this is assumed, such as stating that energy is converted to "kinetic energy" when it heats something. But other times the terms are treated as if thermal energy is not the same, e.g., statements such as "friction converts kinetic energy to thermal energy"; this latter can be treated as a less-formal usage of the phrase kinetic energy to mean "non-thermal kinetic energy".
It is of note that an object's velocity (and kinetic energy) are relative to some other object: neither is an absolute value, and there is some frame of reference in which a particular object's kinetic energy is zero. For example, given an impact of a fast-moving body, the kinetic energy powers a substantial effect (what could be described as an explosion), but if the impact is with another body with nearly the same velocity, the relative velocity between them is small as is their relative kinetic energy, and the effect is small.