Essentially, what this does is break down the microwave spectrum into a series of bands to quickly classify a given emitter. Since the use of specific spectral footprints, (the frequency sets that most RADARs use,) is extremely common most radar systems can be determined by type just by looking at their PRF and operating frequency. This includes the seemingly cryptic designations on the front of radar detectors sold commercially.
Initially the system grouped bands in a somewhat clumsy manner, cutting off at uneven numbers and having multiple sub-designations for what were completely different bands.
The new system has been worked out to distribute frequencies more evenly between bands, thus allowing for faster classification and analysis of a given emitter. Obviously this is of immediate use to those in the ELINT field who require more subtle methods of categorizing a given piece of equipment by the characteristic spectral footprint.

Below is a table of sorts worked out to provide a information on the previous and current frequency band designations. (Note: This applies to mainly to ELINT and military circles.):
Previous Band Designations and Frequencies
VHF: 100-300 MHz
UHF: 300 MHz-1 GHz
L: 1-2 GHz
S: 2-4 GHz
C: 4-8 GHz
X: 8-13 GHz
Ku: 13-19 GHz
K: 19-25 GHz
Ka: 25-40 GHz
Millimeter: 40-100 GHz

Current Band Designations and Frequencies
A: 100-250 MHz (UHF)
B: 250-500 MHz (Upper UHF and lower to mid VHF.)
C: 500 MHz-1 GHz (Mid to high VHF)
D: 1-2 GHz
E: 2-3 GHz
F: 3-4 GHz
G: 4-6 GHz
H: 6-8 GHz
I: 8-10 GHz
J: 10-20 GHz
K: 20-40 GHz
L: 40-60 GHz
M: 60-100 GHz

Sikorsky SH-60B/F Seahawk Basic Avionics Maintenance Review Course; class materials, 1995