ColdFire,
All window functions involve a compromise between steepness of the transition region (region between the passband and the stopband) and sidelobe magnitude. It's like squeezing a baloon; the sharper the transition region, the higher the sidelobe amplitude. The Kaiser window incorporates a parameter that the designer can twiddle to achieve a reasonable complimise between transition band sharpness and sidelobe magnitude. The Kaiser window is based on the prolate spheroid function.
Regards,
Kral