"The song itself has been... criticised for its use of Auto-Tune."
Fragment of a discussion from Comments:'Worst song of all time' becomes YouTube sensation
The really obvious unhuman shifts in pitch of her voice to better match the songs beat, as well as the robotic sort of tone it gives. You get the impression from watching the video that she's pretty much talking the whole way through, and not singing. They use auto-tune to make it seem like she's singing and has great range.