The dynamics of a domain wall in ferromagnetic nanowires under electric current is theoretically studied. The driving mechanism is shown to depend on wall thickness. Thick walls, as in metallic wires, are propelled by the spin torque arising from spin polarization of current, while thin walls, as in nanocontacts and magnetic semiconductors, are driven by the pressure from charge current. The condition for small-current operation, which is a key issue when integrated into devices, is discussed.