A salt formed from either weak acid or a weak base will get hydrolysed. Salts like $NaCl$, $KCl$ and $CaC{l}_2$ are formed from strong acid($HCl)$ & strong bases like $NaOH$ and $KOH$ respectively. Such salts formed from the reaction of strong bases and strong acids doesn't get hydrolysed.
${NH}_{4}Cl$ is formed from a weak base and a strong acid and therefore is an acidic salt.  
${NH}_{4}Cl$ is hydrolysed in water to give [$H^{+}$].
${NH}_{4}Cl$ + $H_{2}O$ ⇔${NH}_{4}OH$ + $HCl$
$HCl\to H^{+}+C{l}^{-}$
Here, $HCl$ is the strong acid formed and $N{H}_{4}OH$ is the weak base obtained.