This YouTube Video is Amy Winehouse singing an acapella version of Rehab. Acapella is a solo performer or a group singing without instrumental sound.
Acapella music originally was, and still often is, used in religious music, especially church music. Jewish and Christian music were originally acapella, and this tradition has existed continuously in both of these religions as well as in Islam.
You can read Amy Winehouse's Rehab song lyrics here.