It comes from using a vocal processor generally referred to as 'auto-tune', although there are quite a few different processors under different names that perform vocal pitch correction (Most notably, the software pitch correction Melodyne). In normal usage, it effectively and transparently allows the producer to correct off-key singing in a recorded vocal. To make more experimental use out of it, producers began making unnatural pitch bends on certain vocal notes for musical effect, (brought into the mainstream attention via the Cher song, thus dubbing it 'The Cher Effect') and eventually this effect was picked up by R&B and Hiphop producers who began using it with more frequency and with more (arguably horrific) drastic application.
Auto-tune processors were originally intended to transparently correct off key sections in a vocal recording, but Cher pulled down the pants of the auto-tune world, T-Pain sodomized it repeatedly, and a vast majority of the recording industry has now forgotten how to make love.
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