HIV (Human Immunodeficiency Virus) is a virus that attacks your immune system. Unlike the flu or other more common viruses, which can be controlled after a brief period of time, your immune system can never totally conquer HIV.
HIV, if left untreated, destroys your immune system. This means that your body is no longer able to defend itself against infections. That's why people with HIV become vulnerable to other illnesses. When this happens, HIV can lead to AIDS.
AIDS (Acquired ImmunoDeficiency Syndrome) is a condition in which your immune system becomes so damaged that it can no longer defend your body against infections. People infected with HIV usually do not have symptoms for several years - even as many as seven to ten years or more. Nevertheless, the virus is at work, slowly destroying your immune system over time.