What can mammograms show besides tumor?

Many things. A mammogram can show normal breast tissue for most people. It can show many benign changes such as fluid filled cysts, calcifications, fibroadenomas. Dilated ducts and dense tissue. These benign changes are far more common than breast cancer.
Many things. The mammogram can show non-cancerous lesions like lymph nodes, cysts and fibroadenomas. The appearance of the milk-producing tissue (fibroglandular tissue) can change depending on your hormonal state. Even trauma, e.g. From a seat-belt injury, can show up on a mammogram. The breasts can also change in the setting of systemic illness such as congestive heart failure.