Thermodynamics is a branch of physics that studies the relations between, heat, temperature, and energy. A branch of math called statistics is often used in thermodynamics to look at the motion of particles.
Thermodynamics is useful because it helps us understand how the world of the very small atoms connects to the large scale world we see everyday.
Thermodynamics also has two main branches called classical thermodynamics and statistical thermodynamics. An important idea in thermodynamics is that of a thermodynamic system.