Tumors, Cysts, etc.
The breast by its very nature is "lumpy-bumpy", often making it difficult to distinguish lumps that count from normal breat tissue.
Lumps "that count" (dominant masses) tend to be distinct from normal breast tissue. They may be fluid-filled (cysts) or solid (tumors)--this can be differentiated by ultrasound. Tumors may be benign or malignant--this can be differentiated by biopsy