When communication occurs over a medium (copper wire, fiber optic cable, radio waves) whose transmitter is simultaneously transmitting multiple carrier signals or frequencies and employs some sort of modulation technique to expand the capacity of the individual frequencies, the communication is referred to as 'broadband'.
Using our string analogy, this would be like connecting multiple strings to the doorknob and then tying the other end of each string to a separate finger and then waggling your fingers. To complete our analogy, we would need waggle each finger in a special way such that the waves are shifted from a regular, repeating pattern, thereby shifting the pattern every so often. This is what modulation does. The string tied to each finger would carry a different pattern of wiggle-waves and beads down the string to the doorknob. Thus, you could send five waves at once to the doorknob instead of one. Each wave would have a different value based on when your finger disrupted the regular signal--you would be shifting the wave slightly.
With any one finger, we can generate a nice, regular wave pattern down the string. If we take that finger and wag it at the 'wrong' time, we can interupt the wave and send a new wave down the wire. This would 'shift' when the wave hit the doorknob. This shifting is one way of modulating a signal. If we were to decide to agree that if you interrupt your wave when it is at the peak, flat or in the trough/bottom of the wave, we would then have 4 different values we could send down the string by shifting our wave one of 4 ways.
- No shift--we keep sending the same wave pattern.
- We wag our finger when the string is at the tall part of the wave.
- We wag our finger when the string is between waves.
- We wag our finger when the string is at the low part of the wave.
This generates 4 possible values we could send on 1 wave. This ability to send 4 different 'symbols' is modulation and this modulation technique allows us to send 2 digital bits of data: 00, 01, 10 or 11.
This resembles the process used in a modulation technique called 'phase shift keying'. If we take each string into account, we have the ability to send 10 bits at the same time by transmitting five specific symbols at a time (each representing two bits).
Unlike baseband, broadband employs not only multiple carriers or frequencies, but utilizes modulation as well.
This is often called "frequency modulation"--using multiple frequencies to carry one stream of data. Broadband systems often use multiple frequencies to carry one stream of data, and they also use modulation techniques to allow the frequencies to carry more data.
See the modulation tutorial elsewhere on this site.