Hi Lei,
FSK is a digital version of frequency modulation. In analog FM the frequency varies continously around a carrier, while in FSK the frequnecy shifts discretly around the carrier. This is all explained in the link I post in my previous post.
Both digital and analog modulation is transmitted by sine waves over the air. The difference between digital and analog signals is how you modulate the sine wave. So for all digital radios the RF transmission itself is done with analog signals which must be rund through and ADC before they are demodulated.
If you are interested in this topic or have more questions I would encourage you to by a book, such as the one TER is linking to, to learn more.
/Fredrik