Ventriloquism is the art of making one's voice apparently issue from something or someone else — a dummy, another person, a dog, or (by a very experienced performer) from thin air.
The voice is never actually "thrown" — that's to say, the performer makes the sounds in his mouth, but by misdirection and an immobile face the listener is tricked into hearing the sound from somewhere else.
This phenomenon is caused by the human senses not having sufficient information to determine the origin of the sound. This is quite striking and can be demonstrated by simple experiments. For instance, blindfold someone and have him try to pinpoint the direction and distance of two coins being tapped together; by changing the position of his hands or covering the coins up to one degree or another, the demonstrator can totally baffle the subject. Another is to line up several people a short distance apart and have one of them make a sound and the others simply mime: from a quite short distance the listeners will be quite unable to tell who's actually sounding off.
It isn't too difficult for the ventriloquist to make most sounds without moving her lips. All the vowels — although one has to be careful with "y" at the beginning of a sound — and most consonants are easy with a little practice. The exceptions are "f", "p", "m", and (not so difficult) "v" and "b". These all require lip movement (thus are called "labial" consonants). The performer either must practise until perfect with these or can (at least at first) substitute one sound for another. "V" can stand in for "f", "b" for "p", and "n" for "m" — the words will sound a little slurred but will be quite intelligible. Having said all this, it's advisable to practise in front of a mirror, as various sound combinations may cause lip movement, or obvious tongue movement, when they aren't expected.
Breath control is important. This isn't just because the performer doesn't want to be visibly gasping for breath while his dummy is chattering away, but also because a smooth, extended release of breath contributes to the quality of the produced sound.
Finally, the tongue and palate can be used to alter the apparent elevation of the sound. The base of the tongue, pressed against the soft palate, allows very little of the voice to get by. This in and of itself produces a muffled sound which can be used to mysterious and striking effect. If then the performer speaks in a harsh, guttural tone with the tongue placed as described, the sound will appear to come from below, somewhere. Speaking in a sharp tone will cause the sound to appear to be emanating from somewhere above the performer. Experimentation will bring out other phenomena.
Training with an expert and tons of practice are necessary to be successful with this art.