The U-Bass or Ukulele Bass is tuned the same as any bass guitar, double bass, or the four lowest pitched strings on a guitar - E A D G (from your chin to your feet).
These beauties have an inbuilt pickup and need to be used with an amplifier, as to achieve the low sound from such a small instrument, the strings are very thick and comparatively loose compared to a treble instrument.
There are now a variety of ukulele basses, some of which have standard wound metal strings, but the thick (up to 4mm diameter) polyurethane strings of the original Kala U-Bass give it a beautiful, mellow, almost classical instrument sound.
The U-Bass is a valuable rhythmic and harmonious accompaniment to any ukulele group.
Want to learn to play U-Bass? Find out about it here: U-BASSics - Ukulele Central's step by step lesson guide