HIV stands for Human Immunodeficiency Virus. HIV is the virus that causes AIDS.
Your immune system is your body's defense system. While many viruses can be controlled by the immune system, HIV targets and infects the same immune system cells that are supposed to protect us from illnesses. These cells are a type of white blood cell called CD4 cells (sometimes called T-cells).
HIV takes over CD4 cells and turns them into factories that produce thousands of copies of the virus. As the virus makes copies, it damages or kills the CD4 cells, weakening the immune system.
Being HIV-positive, or having HIV disease, is not the same as having AIDS. Many people are HIV-positive but don't get sick for many years. As HIV disease continues, it slowly wears down the immune system.
AIDS is the stage where the immune system has become so weakened that there are less than 200 CD4 cells in the system. At this critical stage people may experience opportunistic infections because they are caused by organisms which cannot induce disease in people with healthy immune systems, but take the "opportunity" to flourish in people with HIV. Most of these more severe infections, diseases, and symptoms fall under the Centers for Disease Control's definition of AIDS.