A vocoder is a voice that doesn't even sound discernably human. It sounds like a computer saying words - it mostly sounds like a strange robotic, buzzing voice, and it's often difficult to understand what's being said. See "Mr. Blue Sky" be Electric Light Orchestra, "Intergalactic" by the Beastie Boys, "Mr. Roboto" by Styx, and the way the Cylons talked in the old Battlestar Galactica.
Yay wikipedia! Here's what it sounds like without a melody.
http://en.wikipedia.org/wiki/File:Vocoder_demo.ogg