You can’t talk about form and genre without talking about rhythm, which is the most basic element of music theory. You can write a piece of music without a melody line or without harmonic accompaniment, but you just can’t write a piece of music without rhythm — unless, of course, your “music” is one single, sustained note with no variations in pitch.
Often, the rhythm differentiates one genre from another — such as the difference between alternative rock and punk rock, for example. Put a faster tempo to any Son Volt or Wilco song, and you can file the result in the same section of the record store as the Ramones and the Sex Pistols. Change the inherent rhythm patterns of a song, even a Sex Pistols song, and you can change it to anything from a tango to a waltz. Rhythm is that important to genre.
Rhythm moves through music as a generative force in several different ways. It creates the basic pulse of a song. Rhythm is the part of the song your toes tap along to and your head nods to. Meter helps organize notes into groups using a time signature and defines the repetitive pattern of strong and weak beats that noticeably move a song along. (The individual measures in a piece of music determine the meter.)
This pulse creates a feeling of familiarity and expectation for the listener so that, theoretically, you can throw a pile of unexpected, jangly notes and chords at the listener and still retain a feeling of connection with your audience by holding on to them with the same, steady beat.
The actual rhythm that you hear when you listen to a song is usually referred to as the surface rhythm. For example, often when people say they like the beat of a pop song, they mean they enjoy the surface rhythm, which may simply be a rhythmic pattern on the drums.
Sometimes the surface rhythm matches the underlying pulse of a song (the beat as determined by the time signature that governs the entire piece of music), especially in pop music, where the drums and the bass lines usually follow the basic beat. But sometimes, because of syncopation (which emphasizes the “off” beats), the surface rhythm and the pulse don’t match up.
Tempo comes into play when you discuss the speed of the rhythm of a piece. Is it going to move along quickly and lively or somberly and slowly? The speed at which a piece of music is played determines the overall feeling of the music for the audience. Rarely do you have a super‐happy song played slowly and quietly, nor do you hear a super‐sad song played at “Flight of the Bumblebee” speeds.