Differential equations are equations (expressions) which relate mathematical functions (mappings) to each other or their derivatives. A mathematical function is something like
f(x) = 1 + 2x and a derivative can be understood as the slope of the tangent of the graph of this function, i.e.
f'(x) = df/dx = 2. Differentials are subject to infinitesimal calculus, a mathematical study typically taught in secondary school. Next to linear algebra, they play a fundamental role in many scientific and engineering disciplines. Analog computers are best described with differential equations, which they solve.