A bassist is a musician
whose instrument is (one or both) the double bass
and the bass guitar
Most double bassists also play (and often started out on) bass guitar or electric bass. The opposite is not as common, as there are a lot of bass guitarists who don't play the double bass.
Mayzie claims the bassist is the coolest member of the band, which is naturally true in bands, but in the case of a symphony orchestra (see classical music), there are anywhere from a few up to eight or ten bassists, and they would then be the coolest section. They all have the biggest instrument.