Ferrous fumarate
The fumarate salt form of the mineral iron. Administration of ferrous fumarate results in elevation of serum iron concentration, which is then assimilated into hemoglobin (required for the transport of oxygen), or is trapped in the reticuloendothelial cells for storage. This agent is used as a dietary supplement, and to prevent or treat iron deficiency-related syndromes.
Ferrous sulfate
A sulfate salt of mineral iron formulated for oral administration and used as a dietary supplement, ferrous sulfate is absorbed in the stomach and small intestine and combines with apoferritin to form ferritin, which is stored in the liver, spleen, red bone marrow, and intestinal mucosa. Important in transport of oxygen by hemoglobin to the tissues, iron is also found in myoglobin, transferrin, and ferritin, and is a component of many enzymes, such as catalase, peroxidase, and cytochromes.

National Cancer Institute. NCI Drug Dictionary. Internet. September 15, 2009.