The breast ultrasound works by sending sound waves through your body; based on the different densities of your tissue (how heavy or light a specific area is), the sound waves bounce back to the sensor in different ways. A computer then interprets the results of the bounced-back waves and translates it into a 3-D picture of your tissue.
Radiation is not used to do this test.
Ultrasounds can sense breast changes that can be felt, but cannot be detected by a mammogram. It is also useful for seeing if a lump is fluid filled (known as a cyst, and is not cancerous) or solid-filled, which means that it may require further testing.