What is Hard and soft acids?

A hard acid is a small, highly charged, weakly polarisable Lewis acid ($\ce{H+}$, $\ce{Mg^2+}$, $\ce{Ca^2+}$, $\ce{Fe^3+}$); a soft acid is a large, polarisable one of low charge ($\ce{Cu+}$, $\ce{Ag+}$, $\ce{Hg^2+}$, $\ce{Cd^2+}$). The hard and soft acid–base principle states that hard acids bind preferably to hard bases (oxygen donors, fluoride) and soft acids to soft bases (sulfur donors, iodide).