Although rock and classical may sound like very different types of music, there are many similarities between the two. Rock actually formed as a result of western music; this occurred centuries after western music was created by classical musicians. Rock music would not sound the way it does, had it not been for its predecessors in classical music.
Perhaps the key figure in all of classical music is Ludwig Van Beethoven. He was a rock star a century and a half before rock and roll even existed. Coming shortly after Mozart, Beethoven continued to compose music that was clearly recognizable to any ear. Melody is what Beethoven hammered into listeners. Strong rhythms, simple tunes and easily discernible music were Beethoven's specialties.
Beethoven's music is still used very often today in movies, TV shows and commercials (particularly his 5th and 9th symphonies). However, in the late 20th century, rock musicians used the same techniques. ELO even employed Beethoven's 5th intro to their take on "Roll Over Beethoven." The song starts with the familiar melody played with strings, but then breaks into electric guitar, drum kit and wild vocals. The energy is the same in Beethoven's 5th as it is in the driving rock and roll sounds of the Electric Light Orchestra. This is because they both depend on strong melodies to relay their sound to listeners.
A myth about classical music is that it is boring and slow. Yet, Beethoven's music was anything but boring. With his use of tempo and rhythm, he commanded attention. A good rock song does the exact same thing. The right tempo will make a person turn her head to hear what's going on, and both classical and rock artists can do this.
Rock usually takes a much heavier approach to rhythm, with drummers usually keeping a steady, persistent beat through an entire song. For rock and roll songs, a very blatant rhythm and tempo needs to be present more often. For classical songs, there is always a tempo, but it is usually kept by the conductor. Even though it might not be heard, tempo pushes the music forward steadily and on pace.
The chord progressions in classical and rock are very similar. Both styles usually begin with a hook and then move into a verse or two, eventually reaching a chorus. This is usually a musical journey that is much shorter with rock music, but that is actually the major difference. The build-up toward a chorus happens exactly the same way in both styles, but classical music usually just stretches its verses and choruses out for longer periods of time. A rock song is usually just three to five minutes, while a single movement of a symphony piece is eight or 10 minutes (or more). If one were to play a classical piece in double time, it might sound like a punk rock song in terms of chord progression. Both styles may feature a bridge, too.