Western classical music has for centuries been the standard for what constitutes high musical art. The most famous classical composers bear names that have become as widely studied and mythologized as the Greco-Roman gods. They are canonized and sainted, and their works are considered artifacts of genius; they are approached with almost Biblical reverence. Pop stars gain notoriety more as objects of celebrity; they are worshiped less as artists than as spectacles. We idolize rock stars because we can relate to their success. We can envision ourselves as a Johnny Ramone or a Johnny Rotten, pounding out three chord melodies that rely more on virility than virtuosity. Such is the general perception of the popular musician, a child in a grown-up's body making noise to simply to garner attention, in juxtaposition to the classical image of the eccentric, brilliant recluse pounding out sonatas alone in the dark.
So what then of the musician who aspires both to artistic authenticity and commercial success. Perhaps he listens to classical music and even adores it but know that no one listens to it if it's not a John Williams tune in a blockbuster film. Perhaps too he doesn't want to be bound by the strictures of classical form, and wants to be free to channel the vitality and accessibility of the popular song. What is he to do?
For many pop artists of this description, the answer was to borrow from the instrumentation of classical music. Strings and horns have been a part of the fabric of pop music since its inception; "Rock Around the Clock," widely considered the first rock song, makes more prominent use of horns than it does guitar. From Motown to R&B to Bubblegum pop to Hip-hop to Modern Top 40 rock, these genres have used glossy string arrangements to augment the rhythm and melody of the song. We have been conditioned to think of symphonic instruments as markers of high art and guitars as indicators of something more brash and uncouth.
But across the board, symphonic instruments have been used in pop in the tradition of composers like Beethoven, Bach, Tchaikovsky, Copeland, and other composers who write with a strong sense of melody and sonic order, and who are beloved and widely appreciated for these reasons. More experimental composers like Phillip Glass, John Cage, and Krystof Penderecki whose more open-ended compositions focus more on creating a mood than dictating a melody, the result often being something much more bizarre and frightening than is dealt with in popular music or traditional classical composition.
Radiohead and other contemporary Britpop groups in the 90's both used heavy orchestration in their music, but when we examine their influences we can see how Radiohead's start to diverge from syrupy Beatlesesque arrangements and delve more deeply into the darker atmospherics of a composer like Penderecki. While they never completely abandon the use of soaring, melodic string and horn arrangements, they also utilize techniques of modernist classical composition that simply do not appear anywhere else in mainstream music.
If you listen to songs from the Beatles like "Penny Lane" or "Eleanor Rigby," we see that the string and horn parts serve to elaborate on and accentuate the melody. Songs from Radiohead contemporaries Oasis and the Verve use the same techniques. The soaring violin melody in "Bittersweet Symphony" and the cyclical four note cello part in "Wonderwall" is forever embedded in the consciousness of kids who grew up in the 90's.
Radiohead's earlier work showcases similar techniques. "Nice Dream" uses strings to outline the melody and heighten the emotional impact of the ballad. In OK Computer, however, the group starts to experiment with different techniques. "Climbing Up the Walls," is something we've all fingered as a turning point for the band. After a lengthy string solo, the strings assume a discordant tone that lurks behind the melody instead of augmenting it and works to unsettle the listener. This paves the way for their prominent use of symphonic arrangements in their Kid A/Amnesiac period to create the ominous and chaotic soundscapes that won them such critical and popular acclaim, from the skronking horns on "The National Anthem" to the punch drunk New Orleans Jazz on "Living in a Glasshouse," strings are used to create dissonance and to set a mood, rather than to set a catchy hook.
Jonny Greenwood, the man behind the string arrangement in Radiohead's music, wears his influences on his sleeve. We can see in his soundtrack for "There Will be Blood," full of jarring cacophonous squeals and screeches, that he pays close attention to the importance of using string arrangements to set a mood rather than to simply outline a hummable melody. His prominently use of the Ondes Martenot, an early electronic instrument that produces sounds similar to a string ensemble, is integral to what Radiohead attempts to do with their compositions. "How to Disappear Completely" is a perfect example of how an stacking a series of notes that don't belong in the key on top of a simple two-chord turn can transform a song from a simple ballad to something much more haunting, subtle, and profound.
This being said, Radiohead never abandoned its use of Britpop symphonic arrangements. As we can see with "Faust Arp" and "Harry Patch (In Memory Of)" Radiohead still relies heavily on melodic string parts that are melodically insistent, but continue to mix in a variety of influences and techniques. It is this use of diverse instrumentation that has won Radiohead a label as an "art rock" band. But they have taken their musical exploration well beyond that of any of their peers and created a unique sound, and it is this, not the leaning on classical tropes, that earns them the distinction of "artist."