Teresa, there are quite a few kinds of breast cancer.  Also, breast cancer tumors can differ from each other in certain characteristics such as whether they feed on estrogen.  Some medications work better on the estrogen sensitive tumors, for example.  Then there are the differences in patients that no one can explain.  Even among women with breast cancer tumors that seem exactly the same, two women can have completely different results from the same drug.  So the drug companies keep coming up with new drugs.  This website has information about most of the medications used for breast cancer.  You can type in the name of the medication in the search box to find out more about the ones that interest you.  This link will take you to an overview about medications.