O que é ARCH and GARCH models, volatility persistence?

In an ARCH model (Engle, 1982), $r_t = \sqrt{h_t}z_t$ with iid standardised $z_t$ and $h_t = \omega + \sum_{i=1}^q\alpha_ir_{t-i}^2$. The GARCH model (Bollerslev, 1986) adds lagged variances; GARCH(1,1) is $h_t = \omega + \alpha r_{t-1}^2 + \beta h_{t-1}$ with $\omega > 0$, $\alpha, \beta \ge 0$. Its volatility persistence is $\alpha + \beta$, the rate at which a variance shock decays.