Mammography is the process of using low-dose X-rays (usually around 0.7 mSv) to examine the human breast. It is used to look for different types of tumors and cysts.
Mammography has been proven to reduce mortality from breast cancer. No other imaging technique has been shown to reduce risk, but breast self-examination (BSE) and physician examination are essential parts of regular breast care.
During the procedure, the breast is compressed by a dedicated mammography machine to even out the tissue, to increase image quality, and to hold the breast still (preventing motion blur). Both front and side images of the breast are taken.
See Wikipedia, Mammography.