Mammography is the process of using low-energy-X-rays (usually around 30 kVp) to examine the human breast and is used as a diagnostic and a screening tool. The goal of mammography is the early detection of breast cancer, typically through detection of characteristic masses and/or microcalcifications. Most doctors believe that mammography reduces deaths from breast cancer.
Like all x-rays, mammograms use doses of ionizing radiation to create images. Radiologists then analyze the image for any abnormal findings.

At this time, mammography along with physical breast examination is the modality of choice for screening for early breast cancer.  Ultrasound is typically used for further evaluation of masses found on mammography or palpable masses not seen on mammograms.  MRI can be useful for further evaluation of questionable findings as well as for screening pre-surgical evaluation in patients with known breast cancer to detect any additional lesions that might change the surgical approach, for instance from breast conserving lumpectomy to mastectomy. Wikipaedia.org