What is the difference between an mammogram and a ultrasound?

Imaging: Different imaging studies. Mammogram is usually to see breast tissue. If there is any cyst in breast sometimes ultrasound is ordered to see if this is fluid filled cyst or solid to differentiate.

Screen v. Diagnostic: A screening exam is done on anyone at risk for a disease in the absence of any symptoms. A diagnostic exam is done to evaluate a specific abnormality. Mammograms are effective screening and diagnostic studies; an ultrasound is most useful as a diagnostic test.

Different techniques: They are different technologies/techniques to view breast tissue. They both have important roles in looking at breast tissue.

X-ray vs sound waves: If you have a lump that can be felt, it's likely the radiologist will want an ultrasound as well as mammogram. Mammogram uses x-rays to see calcifications that might indicate a breast cancer, while ultrasound uses sound waves to see different characteristics of breast tissue such as blood flow, size and shape of a lump and whether it's solid or cystic. Ultrasound is also used to guide biopsy.

Mammo vs ultrasound: A mammogram is a low dose X-ray of the breasts. An ultrasound utilizes sound waves to generate images.