Can some one tell me the principles of ultrasound and its medical applications?

I can't find one article that covers everything, but here they have different articles for many different types and uses of ultrasound.

Fetal Ultrasound

Pelvic Ultrasound

Cranial Ultrasound

Abdominal Ultrasound

and more.

WebMD: Ultrasound

Basically, sound echoes are used in place of light reflection to 'see' something. The sound is a higher frequency than humans can hear (3.5-7 MHz).

Thanks so much both of you and for the webb sites!