What does bandwidth mean in general, and in the context of the following examples?
Bandwidth is a term used in describing the analog behavior of a system.  Whenever you have a bandwidth, you have a band -- a range of frequencies upon which you are transmitting.  The bandwidth is the difference between those frequencies.

Specifically addressing your question from the comments, there are practical reasons why we use slices of bandwidth from high frequencies rather than low frequencies.


  Thank you for the answer. In your last paragraph, is there any problem if we assign 0 HZ to 10,000 MHZ to 100 people, each 100 MHZ wide, rather than 20 GHZ to 30 GHZ?


You can do this.  It will work.  However, there are physics reasons not to.  The primary reason you don't allocate low frequency bandwidth this way is that our filters typically operate in terms of relative frequencies.  A simple 1st order lowpass filter will provide 20dB of attenuation per decade.  That means if you have a lowpass filter with a corner frequency of 100MHz, it will theoretically pass 100MHz through perfectly, 1000MHz through with 20dB of attenuation and 10000MHz through with 40dB of attenuation.

If you only wanted to assign a single band to each person, this wouldn't cause too much of an issue.  However, what if those people wanted to share bandwidth?  What if you wanted to have a radio that could tune to different radio stations?  Radio stations require a bandwidth of about 20kHz.  You could assign one of them the band from 20-40kHz, then one to 40-60kHz, the none from 60-80kHz and so on.  If you had 10 such channels, you'd need 200kHz of bandwidth, ranging from 20kHz to 220kHz.  That's about 3.5 octaves.  If, instead you had started the first radio station at 100MHz, then 100.020MHz, 100.040MHz and so on, you would still need 200kHz of bandwidth, but it would range from 100Mhz to 100.220Mhz.  That's only 0.003 octaves!  It's much easier to design an antenna to resonate well over 0.003 octaves than it is to make an antenna that resonates well over 3.5 octaves.

The other big reason to assign higher frequency bands is uniformity.  The effects of the atmosophere and other interactions with the environment are pretty well distributed in a logarithmic space.  Why?  It's the same reason as above: the effects tend to operate proportional to the frequency just like our simple 1st order lowpass filter did.



When designing an RF system, we typically want to find a window where the attenuation is acceptable for our application.  If we need a fixed amount of bandwidth (say 200kHz), its easier to find a window of 200kHz with the same properties in high frequencies than it is to find such a simple window at low frequencies.

One final issue to consider: size.  Lower frequencies means larger wavelengths.  Larger wavelengths means you need larger antennas to efficiently transmit and receive.

One example of low frequency use is submarines.  Submarines are known to have transmitted in the ELF region -- Extreme Low Frequency.  The US Navy ran a transmitter, Seafarer, which operated at 76Hz!  I couldn't find anything on the bandwidth for that particular system, but the transmitting antennas had to be 56km long!  The subs can't even send replies because the transmitting antennas are too big and to power-hungry for the sub!