For such an influential rock album, The Velvet Underground & Nico has very few actual rock songs on it. Truth be told, the record's most important songs are the ones that sound completely different from most rock fare. Perhaps the most famous of those is called "Heroin."
"Heroin" demonstrates some of the more important influences the album had on rock music. John Cale's use of the viola precipitated much of the art/experimental rock scene, serving as bridge between experimental classical composers like La Monte Young and John Cage and the theatrical and experimental rock artists of later decades, such as David Bowie, Talking Heads, and, in turn, more recent acts like Radiohead and Nine Inch Nails. And have you ever wondered when rock became so nihilistically and self-destructively obsessed with themes of drugs and sex? Wonder no more. It is fair (though probably a little over-simplified) to say that it began with The Velvet Underground's debut. As The Velvet Underground's chief lyricist, singer-songwriter Lou Reed spoke plainly about drugs in an unglamorous way with "I'm Waiting for the Man" and "Heroin." Though plenty of artists were writing about drugs and sex before Lou Reed arrived on the scene, Reed's approach to the subject matter—unromantic, explicit, and tortured—was really something new. While "Heroin" has often been misheard as a decadent endorsement of the drug, contributing to a kind of celebration of drug use as an essential part of the rock scene in the following decades, Reed's intention was really more to expose (not endorse) drug use for what it is. The drug trip that we all experience vicariously through Lou Reed in "Heroin" is a harrowing one. The song, both musically and lyrically, was far ahead of its time, and shaped the future of rock music.
What defines The Velvet Underground is the band's sense of experimentation. John Cale's musical education, experience, and style are largely responsible. The various "experimental" tracks on The Velvet Underground & Nico mostly sound that way because of the usual sounds coming out of Cale's viola. Where on one song he uses the instrument percussively, on another he might use screeching atonality, while on yet another he'll create more pleasing and hypnotic "drone" notes.
John Cale's use of drones in "Heroin" and the screeching mayhem he creates at the climax of the song prefigure common elements of today's art rock. A "drone" is a sustained note, going on for a minute, of for the full length of a song, or even for hours on end. Cale's use of the drone in "Heroin" was groundbreaking in pop music at the time, linking the origin of "dronology" in Hindustani classical music and the modern use of "ambient music" in popular music by figures like Brian Eno. (Eno, famous for his work producing for Talking Heads, U2, and Coldplay, is known as the "father of ambient music.") Cale picked up his drone chops in his previous experience as part of The Theatre of Eternal Music. Brainchild of the incredibly influential, Hindustani-influenced La Monte Young, The Theatre of Eternal Music experimented heavily with drones, attempting to sustain single notes for as long as two hours. Cale took what he learned as part of Young's experimental group and injected it directly into Lou Reed's pop-based songs.
The resulting mix was powerful. Eno reportedly said (although the story is probably apocryphal) that not many people bought The Velvet Underground & Nico, but everyone who did was inspired to start a band. Eno's own mid-'70s group, Roxy Music, was directly influenced by Cale's experimental rock on tracks like "Heroin." Eno's Cale-influenced sound has since become a staple of ambient art rock.
Lou Reed's lyrics are just as significant, and worth every bit as much attention as Cale's contributions. "Heroin" breaks with almost all the standard conventions of pop songwriting. Musically, Reed strips down his vocals to the bare minimum of melody. Rhythm, too, is scattered and generally non-musical. Structurally, the lyrics aren't put down in verses of even length, nor do they center on a chorus. This is essentially spoken free-form poetry over music. The closest thing that any other artist was doing at the time was the quasi-rap of Bob Dylan's "Subterranean Homesick Blues"; perhaps unsurprisingly, Lou Reed often drew comparisons to Dylan.
More important than the structure of the lyrics is their content. In the song, Reed speaks plainly—disarmingly nakedly—of his heroin use. When The Velvet Underground & Nico was released in 1966, hard drugs were just becoming the big thing for rock artists. Musicians like Jim Morrison, Janis Joplin, and Jimi Hendrix were all beginning to use drugs heavily, and each was quickly becoming associated with drug use. When these artists did sing about drug use, their descriptions tended to be romanticized or even celebratory—though all expressed later regret for their choices. Lou Reed was different. Like your modern-day slam poets, Reed spoke in the language of the street. And Reed expressed in music what other drug using artists expressed through their tortured lives: the mixed bag of escapism, addiction, and nihilism that is drug use. Reed's in-your-face, unfiltered exploration of heroin use (as in "Heroin, be the death of me / Heroin, it's my wife and it's my life") broke lyrical boundaries in popular music. No one else dared to sing so explicitly about drug use at the time. But even after The Velvet Underground flamed out and faded away (the group broke up in 1972), the nihilistic and honest depiction of drug use stuck around in some strains of pop music. David Bowie, who would later produce some of Lou Reed's solo music, would sing honestly about his drug use in "Ashes To Ashes"—"Time and again I tell myself I'll stay clean tonight"—and many other artists (from The Sex Pistols to Nine Inch Nails) would do the same.