Vibrators are powered by a small motor that spins an off-center weight really fast. Because the weight is off-center, the vibrator shakes. Battery operated vibrators have relatively small motors designed to use the low-current power of a battery. For that reason, they create less powerful vibrations and may have shorter life spans. Battery-powered vibrators are usually designed so that the entire device vibrates, although the location of the motor in the vibrator may determine where the most intense vibrations are.
Plug-in vibrators like the Hitachi Magic Wand use a standard 120V wall current. They have heavier-duty motors, so weigh more, but are also less prone to break down, and produce much stronger and more consistent vibrations. They're often designed so that an attachment or head is the primary point of vibration.