The Kardar-Parisi-Zhang (KPZ) equation describes stochastic interface growth: ∂h/∂t = ν∇²h + λ(∇h)² + η. The nonlinear λ term breaks up-down symmetry and drives the surface toward KPZ universality class with roughness exponent α=1/2, growth exponent β=1/3 in 1+1d. Compare with linear Edwards-Wilkinson (EW, λ=0).