Autotune vs Vocoder

An autotune is a plug-in for a correction that is most commonly used for editing a person's (usually a singer's) pitch. However, a vocoder is a specific effect that usually combines a person's vocals with synth-notes. Despite their difference, they both can be used to produce a similar effect of making a person's voice sound somewhat artificial in a song. For example, the song "Believe" by Cher had an autotune effect in certain parts towards the chorus while "Hide and Seek" by Imogen Heap used a Vocoder throughout most of the song.

Autotune (section from "Believe"):

Vocoder (section from "Hide and Seek"):