HIV (human immunodeficiency virus) is the virus that causes AIDS. This virus is passed from one person to another through blood-to-blood contact (blood transfusions, HIV-infected needles) and sexual contact. In addition, an infected pregnant woman can pass HIV to her baby during pregnancy or delivery, as well as through breast-feeding.
AIDS (acquired immunodeficiency syndrome) occurs when the HIV infection has weakened one's immune system to the point that it has difficulty fighting off certain illnesses and infections. "Opportunistic" infections also occur, taking the opportunity a weakened immune system gives to cause illness.