Ventriloquism is an ancient art which, as it is practiced today, combines misdirection (also used in stage magic and slight-of-hand), commonly, but inaccurately, called "throwing your voice," with the use of knee figures (such as those used by the late Edgar Bergen and Paul Winchell) or puppets (which are preferred by Shari Lewis).
Typically, the ventriloquist plays the "straight man," and the knee figures ("dummies") assume the role of the comic. I have often joked that ventriloquism is perhaps the only field in which the practitioner willfully cultivates multiple personalities.
Ventriloquism's objective is to convince an audience that the knee figures are, in fact, real and not merely creations of the imagination of the mind of the performer. Contrary to the common term "throwing your voice," there is no "magick" (to use thelemist Aleister Crowley's spelling), in the literal sense of bewitching, involved. Rather, an illusion is created through the skill of the ventriloquist.
First, the audience is already on the side of the ventriloquist (or perhaps of the knee figure or puppet). They truly want to believe that the little one is real.
Second, the ventriloquist needs to feed the illusion through a combination of skillful manipulation of the figure or puppet, control of her or his own lip movements, and keen character development (so that the personality of the her or his sidekick, or sidekicks, is notably different from that of the ventriloquist).
Perhaps the most talked about skill involves limiting the ventriloquist's lip movements. While lip control is not completely necessary (as anyone who has seen Edgar Bergen's classic films will vouch for), there are various ways of effecting it. The means I have always utilized is consonant substitution. Certain sounds (especially b, p, m, f, and v) cannot be said, or can only be said with considerable difficulty, without moving one's lips. So I will say "d" (some use "g") while thinking of "b," will say "t" (some use "k") while thinking of "p," "n" (some use "ng") while thinking of "m," a soft "th" while thinking of "f," or a hard "th" while thinking of "v".
Above all, ventriloquism is fun. If you are at all interested in the art, and have some time, I would encourage you to learn. Although higher-end knee figures are not inexpensive, there are many reasonably priced puppets (and even some knee figures) you can purchase which will probably not put too much of a strain on your pocket book. (Checkout some of the links.)