Techniques

Human voice is usually considered to have at least three voice registers; ranging from lowest to highest, they are the: chest register, head register, and falsetto. (The whistle register, comprising the highest notes that a human voice can reach, is also often considered a "full" register, although individuals who are able to use it well are fairly rare.) Some singers choose to remain in a single range (usually the chest register) throughout a piece, but many will switch between these different ranges in order to produce a wide range of pitches, or even simply for effect.

Yodelling is a technique that requires rapidly switching between at least two different registers many times in the same phrase, producing a distinct high-low-high-low sound.

Vibrato is a technique used by singers (and many instrumentalists. For instance, string instruments that are played with a bow can produce vibrato tones) in which a sustained note actually wavers very quickly and consistently between a very slightly higher and a lower pitch, giving the note a slight quaver. Vibrato is the pulse or wave in a sustained tone.
Singers use vibrato as a means of expression. Many successful artists have built a career on deep, rich vibrato ability.

A melisma occurs when a singer switches pitch while singing the same syllable. It is used heavily in baroque vocal music, as well as to a somewhat lesser extent in popular music.

A vocal warm-up is usually required before the vocal chords are expected to perform at its full potential. Proper breathing technique is also a key factor in singing correctly.