Instead of 6 it has 4 strings, (although 5 are also common and 6,7,8,14 or 17 are possible) and it is two octaves lower. The function of the bass is to provide a link between drum and guitar supporting both melody and rhythem. The most popular bass is the fender jazz. The bass is at it's best in Jazz and blues, but it its an unmissable adition to any band.
The many techniques that can be used on bass also make it a totaly different instument than the regular guitar. As opposed to cords and single note play on the regular guitar (both of which can be used on bass) a bass can be played with a pick with the fingers (in many different ways- the most popular being the walking fingers method)and probably the most recogniseable, the pop and slap. This technique is best observed in the music of "Level 42" (note the link to the guide). In this legendary band the singer/bassist Mark King pops and slaps so much that it dominates over the guitar- a rare feat in pop music.
Other famous bassits include Sting, sir Paul McCartney, Jaco Pastorius, Flee and many others.