Perhaps the most basic of all pop song structures is the Simple Verse form, in which a section (verse) is heard over and over again with little or no alterations or development. The lyrics might change (or a solo might replace the vocals entirely), but the music itself (i.e. the underlying chords and fundamental harmonies) remain constant with each iteration.
Obviously, the Simple Verse form puts structural emphasis on the verses because no other sections exist. The limitation of the Simple Verse form, then, is that using the same musical material over and over again can easily become monotonous. The solution is executed in two primary ways: with middle 8s and choruses.
The AABA form also puts structural emphasis on the verses, but it adds a middle 8 to provide contrast to (and thus enhance) those verses. A design most closely associated with Tin Pan Alley, AABA form is when the verse (A) is immediately repeated (AA), then the middle 8 is heard (AAB), before the verse is repeated once more (AABA). Usually these four sections will not last long enough to justify a complete song, and consequently it is often repeated in whole (AABA with full reprise), or in part (AABA with partial reprise).
Another subset of the AABA form can be found where the initial AABA components are heard again, but (unlike the full and partial reprises) in an unpredictable combination of A and B sections. This is called the Broken AABA form.
In contrast to the AABA design and its various permutations, the Verse-Chorus form employs a chorus to contrast with the verses (instead of a middle 8). Where the Simple Verse and AABA forms all place structural emphasis on the verse, the Verse-Chorus forms place emphasis on the choruses.
There are three basic subsets of the Verse-Chorus design: 1) Simple Verse-Chorus is when both verse and chorus use the same underlying chords, but incorporate different melodies, lyrics, textures, or energy levels; 2) Contrasting Verse-Chorus is when the verse and chorus use very different music elements; and 3) the Beatles' Contrasting Verse-Chorus is a subset of the Contrasting Verse-Chorus in which there are two different versions of the chorus.
Covach, John. "From 'Craft' to 'Art': Formal Structure in the Music of the Beatles". In Reading the Beatles: Cultural Studies, Literary Criticism, and the Fab Four, ed. Kenneth Womack and Todd F. Davis. State University of New York Press, Albany, NY, 2006.