The Bass Guitar is a relatively recent instrument, being invented after the Electric Guitar. It is normally a 4 string fretted instrument although 5 and 6 strings basses exist, as do fretless examples. The first to be commercially produced were Fenders (Precision Bass). The Bass is tuned the same as a Double Bass - EADG - the same as the lowest 4 strings on a Guitar. A fifth string is normally tuned to B below the G where fitted although C above G is used as well.
Being Electric they use PickUps like a normal Electric Guitar.
Acoustic versions of Bass Guitars exist but are rather quiet in normal circumstances.