It is a virus that attacks the human immune system. Someone infected with the virus can live with HIV or be HIV positive for many years without becoming ill or showing symptoms. During this time however, HIV remains in the body damaging the immune system and the person remains infectious; able to spread the virus to others if a few simple precautions are not follwed. HIV Virus
Over time, HIV can damage the immune system to such a degree that infections may begin to occur as a result of a weakened immune system. Eventually, one may acquire various illnesses due to the damage done by the virus.
When this happens this is called AIDS or Acquired Immune Deficiency Syndrome. That is, a collection of illnesses.