what is the bandwidth of electromagnetic spectrum?

Relevance

I think you mean to ask what is bandwidth for EM waves. Spectrum implies all EM wavelengths from Plank's length at one end to the diameter of our known universe at the other.

A few years ago, a guy named Fourier found that all waves were made up of multiple waves. That is, we can simply add up all the faster frequency included waves (called harmonics) to get the same result as the fundamental wave they are in.

Well, each of these harmonics can carry information (0 or 1 for example). So the more of them we can include in our signal measured by the fundamental wave, the more information our signal can carry. One measure of the width of that wave is called bandwidth, which is simply the range of frequencies in the harmonics. The wider the bandwidth, the more information a given signal can carry. Check out the definition:

"(1) A range within a band of frequencies or wavelengths.

(2) The amount of data that can be transmitted in a fixed amount of time. For digital devices, the bandwidth is usually expressed in bits per second(bps) or bytes per second. For analog devices, the bandwidth is expressed in cycles per second, or Hertz (Hz).

The bandwidth is particularly important for I/O devices. For example, a fast disk drive can be hampered by a bus with a low bandwidth. This is the main reason that new buses, such as AGP, have been developed for the PC. " [See source.]