Hydroxyurea, or more properly Hydroxycarbamide is a pharmaceutical used in chemotherapy. It's more toxic to abnormal cells than normal ones and, as such, can be used to treat several conditions including polycythemia vera, sickle-cell anemia, AIDS, psoriasis, mastocytosis, and some forms of leukemia.
It has the usual side effects associated with chemotherapy, including nausea and digestive problems.
The drug's mechanism of action is poorly understood, but it's believed it suppresses the activities that result in cell division at the level of DNA.