Multiway speakers (i.e. speakers that have more than one drive unit) have a crossover circuit which splits the frequencies of the signal to go to the appropriate drive unit. Most drive units have quite a limited range of frequency response. So large drive units (8" or above) tend to be good for bass frequencies but aren't fast enough to do treble frequencies. Treble units (usually around 1" in diameter) are fine from about 2.5khz up. The crossover takes into account the way the drive units work and the designer chooses appropriate values to split the signal between them.
Generally, the more drive units you have, the more complicated the crossover. There are ramifications to this of course (as with anything in life) and crossovers have the nasty habit of really messing about with the phase of various parts of the signal. they also absorb a great deal of power and waste it in heat. Some very few speakers are designed with just one so-called full range drive unit which means that no crossover is required. These usually lack extension in the frequency extremes (bass and treble). However, they make for a very immediate sound. Their only other limitation is often power. They can be quite delicate and can often be driven with quite modest amplifiers.