I'd divide Dylan's career not by sound, but by theme; simplistic, and not without overlap (or outright oddities), but here goes:

Political (1962-65)
For which he made his name, and besides his first album being covers of existing material, his first four albums thereafter all come under this period.
Bob Dylan - Bringing It All Back Home

Abstract (1965-1967)
In which he seems most comfortable making obscure literary references, paying homage to TS Eliot and telling sweet fables such as "Frankie Lee and Judas Priest".
Highway 61 Revisited, John Wesley Harding, Blonde on Blonde

The "Down" Period (1969-1973)
I don't mean "down" in a negative sense, and he's certainly still very prolific, but there's definitely a loss of distinctive originality or creativity, though, I would argue, it's still very self-consciously so. From the smiling Dylan on the front cover of Nasvhille Skyline to the mish-mash of tunes and oddities on Self-Portrait, which isn't really a self-portrait at all (though it's a nice album cover).
Nashville Skyline - Dylan

Love-torn (1973-1975)
Despite The Basement Tapes with The Band in between the three albums which comprise this period, this is my favourite of Dylan's periods, and the one in which he is most consistent since his political period.
Planet Waves, Blood on the Tracks, Desire

Christian (1979-1981)
Street Legal in between Love-torn and Christian; this is the one in which Dylan found God, and as Omar once said, even if you don't believe in God, you don't doubt that he does.
Slow Train Coming, Saved, Shot of Love

1980s (1981-1990)
Seems a bit silly to call a period 1980s, but since this is the one I am least familiar with, it seems adequate enough right now. He's still very, very prolific, and still very, very good, though nowhere near as consistently so... and it's an interesting departure from his sixties stuff and his recent rejuvenation.
Infidels - Under the Red Sky

Reflection (1992-present)
The longest of the periods, I think, by some distance. There may be arguments to be made for a division after World Gone Wrong, since his last three albums seem to work together as a cohesive whole, but for now, I'll keep it as this until I revisit it. Time Out of Mind is one of his best albums. Anyway, in some form or another, he seems to be revisiting previous themes and reflecting on his own career, sometimes obviously, sometimes not.
Good As I Been to You - Modern Times

Any thoughts and revisions welcome.