How does breast cancer appear like on a mammogram?

Quite variable. Breast cancer can appear as a spiculated mass, cluster of tiny calcifications, smoothly marginated mass, area of subtle distortion or be invisible on mammogram. Some tumors are only seen on ultrasound and a few are only felt by you or your doctor. The point is that the appearance of tumors on mammogram is quite variable.
Crab like mass. Irregular, with multiple crab like projection.
Density or Ca++. Mammograms will often detect areas that are more dense than normal breast tissue or that have little flecks of calcium within milk ducts. These may be categorized based on risk, with those that are deemed suspicious warranting a biopsy. Of those abnormalities that are biopsy-worthy, about 15-30% end up being cancerous.
Multiple ways. Breast cancers can have multiple appearances: round mass, irregular mass, asymmetry, distortion of normal architecture, microcalcifications, skin thickening, nipple retraction, skin retraction, abnormal lymph node. .