You may find clearer explanations if you take a step back from EM waves, and consider the properties of a one-dimensional time varying signal (which, e.g., can be generated by measuring voltage induced across an antenna with an EM wave)
The property 'frequency' describes a fundamental property of a sinusoid signal - how often it cycles per second. Using a Fourier transform, any signal can be represented as a sum of different sinusoids. The bandwidth of a signal describes the difference between its maximum frequency and its minimum frequency.
If multiple signals share frequency components it can be very difficult to separate them. One solution to this problem is to modulate the signals around a 'carrier' frequency (AM radio, amplitude modulation, is the simplest example of this). This moves the signals to different parts of the frequency spectrum so they can be easily separated.
The carrier frequency must be greater than the original signal bandwidth, and the separation between two different carrier frequencies imposes a limit on the bandwidth of signals that can be transmitted without interference.
The bandwidth associated with a particular frequency is either a) a measurable property of a signal being transmitted or b) (I think this is what you are getting at) a decision by a regulatory body such as the FCC to create a scheme in which people can share the EM spectrum without stepping on each other.
Bandwidth and frequency are measured in the same units i.e. Hertz.
Bandwidth is the difference between the highest and lowest frequency transmitted over a channel.
With this definition, it is clear that the bandwidth cannot be larger than the highest transmit frequency.
Usually the bandwidth is much, much smaller than the transmit frequency and is sometimes given as a percentage.
When the FCC or other regulatory body allocates portions of the spectrum for use, they specify many things, including the allowed bandwidth.
Let's take some examples, AM radio stations in the India operate between 520 kHz and 1610 kHz, with a channel spacing (bandwidth) of 10 kHz. You can put 109 different channels in that band. 10 kHz is fine for talk radio and news but not great for high fidelity music. The FM band operates at more than 100 times the frequency, between 88 MHz and 108 MHz, with 100 channels and a spacing of 0.2 MHz (200 kHz) between channels. With 20 times the bandwidth, there is room for high quality stereo audio (plus guard bands to minimize interference, pilot tones, and other things). If you tried to do this in the AM band, there would only be room for five channels. Standard analog TV requires about 5 MHz per channel, so when the need arose for more than the original 13 channels, they had to go up another factor of ten in frequency, with UHF stations up to ~800 MHz.
