Heavily influenced by The Beatles, The Easybeats, The Who, The Kinks, Power Pop was a style that first appeared in the very late 1960s with bands like The Rasberries and Badfinger. Vocally driven rock music that has very poppy melodies and vocal stylings. Guitar riffs are prominent, as is the drum track.
The term is common for the music of the British 1979 Mod revival (Secret Affair, The Chords, Lambrettas, early Jam). That era's anthem may very well be "Time for Action" by Secret Affair (see Youtube link)
U.S.-American The Knack, who where similarily 1960s-influenced (but unrelated to the "Mod" movement) had a massive hit with "My Sharona" in 1980, a well-known prime example of the style.