What is the difference between unconfirmaty and a hiatus?

An Unconformity is where there is a discontinuity in the column of material, with a different style of material taking over after the discontinuity.

The place where gravels and silts abut solid rock would be a simple example of an unconformity.

A Hiatus is merely where the same type of material exists on both sides of the discontinuity. The column of material continues, but there is a time slice missing.