RGB : the Red-Green-Blue value of a Color. RGB values are integers (byte values) from 0 to 255. So RGB is a three byte value with 256x256x256=16,777,216 possible values.

HSB : the Hue-Saturation-Brightness is a different way of indication of color. Values are measured in floating-point from 0.0 to 1.0 .

There are mathematical algorithms in converting between RGB and HSB.

And here is the java source code for those who are interested...