O que é SABR model?

The SABR model (stochastic alpha, beta, rho) describes a forward $F_t$, driftless under its forward measure, and its volatility $\alpha_t$:

with an exponent $\beta\in[0,1]$, a correlation $\rho$ and a volatility of volatility $\nu$; the initial value $\alpha=\alpha_0$ is the fourth parameter.