AIDS stands for Acquired Immune Deficiency Syndrome:
AIDS is caused by a virus called HIV, the Human Immunodeficiency Virus. If you get infected with HIV, your body will try to fight the infection. It will make "antibodies," special molecules to fight HIV.
Tests for HIV look for these antibodies in your blood or mouth lining. If you have them in your blood, it means you have HIV infection.
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.
You don't actually "get" AIDS. You might get infected with HIV, and later you might develop AIDS. You can get infected with HIV from anyone who's infected, even if they don't look sick and even if they haven't tested HIV-positive yet.
Most people get the HIV virus by: