AIDS really just means a more advanced stage of HIV.
Healthy, non-HIV+ adults have a cd4 count of between 500 and 1,500. When a person with an HIV-weakened immune system has a cd4 (or T cell) count below 200, or if that person comes down with one or more rare 'opportunistic infections', he may be diagnosed by a doctor as having AIDS.
"AIDS" stands for "Acquired Immune Deficiency Syndrome." The "syndrome" part means that AIDS is not a single disease but a collection of diseases. Proper treatment, however, can prevent HIV from turning into AIDS and can restore the health of people with AIDS.
Of the over 1 million people living in the U.S. today with HIV, most do not have AIDS.1
1U.S. Department of Health & Human Services