A combo is, as the name suggests, a combination amp - meaning that it has both the amplifier and speaker within one unit.
Excellent, half the amount of gear to lug around! One thing a combo is not, though, is a compromise - virtually all the major manufacturers of the bass world produce combo versions of their amps.
Combination Bass Amps
Generally combos tend to have a lower maximum power output than a stand-alone amp. But if you don't need to be able to deafen an entire bingo hall from 800 metres, then a combo could be the perfect choice - easier to store, easier to transport and easier to set up -- and without sacrificing a shred of sound quality -- they're just as capable of handling rehearsals and medium-sized gigs as an equivalent stack. Plus they're usually slightly cheaper - never a bad thing!