The art of bringing a character
to life using only your voice, either because that's the only aspect
of your medium
s) or because the visual
aspect of your character is being handled in another fashion (usually through animation
Voice acting isn't the same as overacting, though a touch of melodrama never hurts. Neither is it the same as "making funny voices", though lots of voice actors can do that.
It is a common misconception that anyone who can act can also voice act. This is not true; typically a voice acted character must express a more robust range of emotions. When an actor performs as themselves (as a guest star on The Simpsons, for instance) they can tend to come off flat and unconvincing for this reason.
And there are people who are only so-so in front of the camera but shine like a jewel in front of the microphone. Mark Hamill is the prime example of this; while none of his on-screen appearances have been more than serviceable, he has proven to be one of the best voice actors in the world, creating many memorable charactors for animated movies and computer games, none of which sound like any of the others. Most famous for giving us our first real Joker in Batman: The Animated Series, he also shone as the oily, bass-voiced villain Adrian Ripburger in LucasArts' classic computer game Full Throttle. Even his cameo appearance as the Evil Cat on The Powerpuff Girls was memorable.
Other superb voice actors:
Tress MacNeille: This woman is everywhere. Most famous for playing Babs Bunny on Tiny Toon Adventures and Dot Warner on Animaniacs, she also regularly does guest voices on The Simpsons, Futurama, and just about every animated children's show currently running.
Kevin Conroy: Just as Mark Hamill gave us our first real Joker, so did Kevin Conroy give us our first real Batman. His ability to modulate his voice from a deep bass for Batman to a higher register for Bruce Wayne harkens back to the days of Bud Collyer as Superman.
Charlie Adler: Though he's doing more directing than actual voice work these days, Charlie Adler's work is just as pervasive as Tress MacNeille's, beginning with his work as Buster Bunny on Tiny Toons.
Jim Cummings: What can you say about a man who is the new official voice of Winnie the Pooh one minute, and the villain of half-a-dozen other movies the next?
Frank Welker: A master of animal as well as human voices. Did you hear a dog bark or bird chirp in an animated movie? Odds are good it was Frank Welker.
There are many more talented people out there doing phenomenal work with just their voices. Let's give them the credit they deserve.