Familiar jazz songs are sometimes arrangements of songs from other styles of music. When jazz singers create an arrangement of a musical theater standard, they usually change the notes and rhythms from the original music. Jazz singers create their style with rhythmic flexibility, and the singer and pianist don't always have to be together note for note (called back phrasing).
Sound: The world of jazz is similar to the other contemporary fields in that the singers have to make a variety of sounds. The sounds in jazz are often more about using the voice like a musical instrument than making big vocal sounds like you hear at the opera. The singer often sings syllables or rhythmic sounds in place of words.
Healthy technique: Jazz singers need to have a good ear, because their music is often improvised and changes with each nightly performance. The singer also has to know how to scat, which is using doo-wop kinds of syllables, while singing a variety of notes that may or may not be written on a page of music.
The jazz singer needs a great sense of rhythm, because the musical instruments are often playing background music while the singer offers his own melody line.
Naming names: Some jazz singers who apply great jazz technique include the famous scat queen herself, Ella Fitzgerald; the laid-back, sultry sounds of Diana Krall; and the man who uses his voice like an instrument, Bobby McFerrin.