What are breast lumps composed of?

Breast tissue. Breasts are inherently lumpy structures. The milk producing portions are branch like and separated by fat. Younger women have more of the glandular breast tissue and are often more lumpy on examination. Any lump that stands out from the rest if the breast tissue should be evaluated. Some "lumps" are normal, some are cysts filled with fluid, some are benign growths and some are cancerous.
Breast lumps. It depends: 1. Cysts are fluid; 2. Tumors are solid; 2a. Benign tumors often are composed of the supportive tissues of the breast [fibroadenoma]; 2b. Malignant tumors are typically from the milk producing tissues; 3. Finally fibrocystic lumps are just normal lumpy breast tissue made up of all elements of breast and often vary with the menstrual cycle.