Whilst there are subtle differences between the Usuba and the Nakiri knife the main difference is that the Nakiri knife is double beveled - both knives are made to slash vegetables.
Usuba or thin blade knife. In Japanese 'usui' means thin and 'ha', as in hamono, means blade. In Japan you're more likely to see an usuba in the hands of a professional chef. Along with the deba and yanagiba this knife is one of the three main knives used in a Japanese commercial kitchen. The Usuba is the thinnest of the three general knife shapes. Flat edge profile and square tip. Used for push cutting, rotary cutting thin sheets and cutting thin strips from those sheets.