Fibroadenoma: These round, hard noncancerous lumps usually occur near the nipple and feel smooth and solid like a marble – and move like one too. Their cause isn't known, but they're common in teenagers and young women, says Saslow.

If you're young, fibroadenomas are usually left alone. "But in middle-age and older women, they're often surgically removed just to make sure there's no cancer there," she says.

Cysts: Among the most common causes of breast lumps, these harmless, fluid-filled sacs typically occur in women 30-50 years old. They can come and go with your period and usually aren't dangerous.

"When close to the breast's surface, cysts feel smooth on the outside and liquid on the inside," Saslow explains. "[Deep] cysts will feel hard."

They're often diagnosed by ultrasound and/or by fine needle aspiration. During the outpatient procedure, a thin needle is inserted into the lump to remove fluid for testing.

Intraductal papilloma: These small, difficult-to-feel but harmless growths are found inside milk ducts. They frequently occur near the areola (the darkened skin around the nipple) in women 35-55 years old. In some cases, their only symptom is a watery, pink or bloody discharge from the nipple, which is why you should see a doctor.

"Nipple discharge [is also] a sign of breast cancer," Saslow says. "The evaluation may be just imaging, but usually includes a biopsy."

Cancer: Most cancerous lumps occur in women over 40. Although the disease can strike any part of the breast, it often lodges in its upper quarter, including near the armpit.

In rare cases, other symptoms – redness or dimpling of the skin, an inward-turning nipple or discharge – indicate inflammatory breast cancer, a rare but fast-growing cancer, she says.