The human singing voice is split into 4 distinct registers.
(1) Vocal Fry (the lowest)
(2) Modal
(3) Falsetto
(4) Whistle (the highest)
Each of these four registers has its own vibratory pattern, its own pitch area (although there is some overlapping), and its own characteristic sound. Below I will provide a short summary of each register with an audio example.
The vocal fry register is the lowest vocal register and is produced through a loose glottal closure which will permit air to bubble through with a popping or rattling sound of a very low frequency. The chief use of vocal fry in singing is to obtain pitches of very low frequency which are not available in modal voice. This register is not used that often in singing, and both men and women have been known to do so.
Modal:
The modal voice is the usual register for speaking and singing, and the vast majority of both are done in this register. As pitch rises in this register, the vocal folds are lengthened, tension increases, and their edges become thinner. A well-trained singer or speaker can phonate two octaves or more in the modal register with consistent production, beauty of tone, dynamic variety, and vocal freedom.
Falsetto:
The falsetto register lies above the modal voice register and overlaps the modal register by approximately one octave. The characteristic sound of falsetto is inherently breathy and flute-like with few overtones present. Both men and women can phonate in the falsetto register. The essential difference between the modal and falsetto registers lies in the amount and type of vocal cord involvement. The falsetto voice is also more limited in dynamic variation and tone quality than the modal voice.
Whistle:
The whistle register is the highest register of the human voice. The whistle register is so called because the timbre of the notes that are produced from this register are similar to that of a whistle or the upper notes of a flute, whereas the modal register tends to have a warmer, less shrill timbre. Women of all voice types can use the whistle register. With proper vocal training, it is possible for most women to develop this part of the voice.
Listen to Brett Manning demonstrate all 4 registers, as he hits notes in 6 octaves below.
