Vocals, or vocalization, refers to the basic act of making musical sounds with the human voice, as opposed to doing the same by using another instrument such as a horn or string, or by using other aspects of the human body such as clapping hands, stamping feet, etc. Vocalization is one of the oldest forms of musical expression, and can be found in every human culture.
Most musical vocalization refers to the more specific act of singing; that is, to generate sustained musical tones, especially when synchronized with syllables of speech to create a sustained, tonal representation of words different from spoken word. Other forms of vocalization include spoken word itself, sometimes rhythmically arranged into styles like "rap", as well as other human sounds such as screams, grunts, and other sounds produced by the mouth or vocal chords.
As the term is used in popular music production, "vocals" refer primarily to melodic or harmonizing lines calling for the human voice, as opposed to other instruments common in popular music such as guitar, bass, drums or keyboards. Vocals are typically further subdivided into "lead" and "background", with the lead vocalist singing the melody line and words of the song, while background vocalists support the chord structure of the song, or provide major harmonies or countermelodies to the lead vocalist.