There are basically four HIV stages which lead to the "AIDS" or "Acquired Immune Deficiency Syndrome" disease.
When a person is infected with the "HIV" or "Human Immunodeficiency Virus", the T helper lymphocyte cells which are a critical part of the immune system are essentially attacked by the HIV virus killing these cells which greatly reduces their number over a period of time.
When these T cells drop below a certain level this is when the individual is diagnosed with having AIDS.
The first of the four HIV stages is known as the "Primary HIV Infection".
This is the stage which occurs shortly after the individual has become infected with the HIV virus and may appear as flu like symptoms and last for only a few weeks which is the primary reason why many people do not even realize that they have the disease as they believe it to be simply a form of influenza.
The second stage of HIV is called the "Clinically Asymptomatic Stage" this stage lasts on average for a period of ten years and in most cases mainly affects the lymph nodes causing swollen glands with very few other symptomatic problems.
The third of the HIV stages is called "Symptomatic HIV Infection" which occurs when the HIV virus has broken down the immune system to the to a critical level in which the individual no longer has the resistance to certain types of cancer or infectious bacteria that they would normally have.
The fourth and final stage of HIV is basically the development of AIDS where the HIV virus has significantly damaged the immune system to the point where the individuals T cell count has dropped below a certain level or in some cases AIDS is diagnosed if they have developed one or more of a certain type of cancer or severe infectious bacteria.