What is the difference between a perfect cadence and a plagal cadence?

Although both a perfect cadence and a plagal cadence resolve to a I chord, they have very different effects. A perfect cadence resolves V-I, while a plagal cadence resolves IV-I. The vast majority of art music resolves using cadences from V, while the plagal cadence is most common in hymns. For this reason, it is often colloquially called the "Amen cadence". The plagal cadence is uncommon in tonal music as it does not establish tonality as clearly as a movement from V to I does. Even in hymns, a plagal cadence often follows a perfect cadence!