It's a vocalising effect, a style added to singing, usually to make a plain voice sound a little more interesting. It's very effective if used sparingly, especially as an echo effect, with an upward inflection added at the end of a sustained note.
But if the singer tries to force it into every lyric it begins to sound very odd, and they can also come unstuck. Janet struggled in her uptempo song a week or so ago when she lost the beat because of the delay caused by her forcing the effect into each rapidfire lyric, and much the same thing happened with her US equivalent, Drew, in X Factor USA last week, where for the first time that girl sounded untidy in places.