Tempo is the speed of a piece of music. Tempo plays an important role in the overall feel of a song, affecting the flow of melodies, phrasing, and rhythm. For example, a song with a very fast tempo will restrict the intricacies of a drum part. Different tempos are more appropriate for different styles of music. Slow ballads like Elton John's "Candle in the Wind" and Whitney Houston's "Greatest Love of All" wouldn't have the same effect if they were recorded with very fast tempos. Thrash metal bands, however, rely on fast tempos for the aggressive nature of their music.
A melody is a series of notes that changes in pitch and duration to make up a unified whole. In a pop song, for example, the melody is the most prominent part of music, the catchy hook that sticks in the listeners' head. Melody plays different roles across different genres of music. In many folk songs, for example, the melody is static, placing more emphasis on the lyrics than to any sort of catchy tune. In funk music, melody is superseded by the rhythm and groove. Classical music, on the other hand, often features very complex melodies.
The rhythm in a piece of music is the timing and spacing between notes being played. For example, in jazz music, the rhythm is often syncopated, meaning the accent is on the off beat, giving the music a "swing" feel. Unlike tempo, which usually stays consistent during a piece of music, rhythm often changes between different parts of a song. A common change in rhythm in a pop or rock song is to switch to half-time in the chorus to give the section a grander feel. Much of reggae's sound relies on its unique one-drop rhythm.
Dynamics are changes in intensity and volume throughout a piece of music. Dynamics play an important role in moving a piece along while keeping the listener interested. In pop, hip-hop and rock music, musicians often add extra instrumentation and harmonies in the chorus to accentuate the importance of the section and to create energy. A classical piece or a heavy metal song may change in dynamics numerous times throughout a piece, alternating between moments of tension and release.