Being diagnosed with AIDS used to be considered a fatal condition for most people.
Since the invention of antiretroviral drugs, however, an HIV infection has become more manageable.
HIV is a retrovirus, or an RNA-based virus, that uses a protein called reverse transcriptase to turn itself into DNA, which can then mix into a host cell's DNA. HIV then reproduces and destroys the host before moving on to infect other cells. What makes HIV so deadly is that it attacks cells in the immune system, rendering those infected defenseless against disease.
Antiretroviral drugs are successful because they target the proteins and other mechanisms that HIV needs to turn its RNA into DNA. When it's unable to do this, HIV can't multiply. A virus that can't reproduce itself does much less damage than a virus that can.
HIV is a tough virus to stop, though. As it replicates, HIV tends to mutate frequently. For this reason, a drug that works against one strain might not work very well against a mutated strain. That's why it takes a combination of antiretrovirals to neutralize HIV. Some antiretrovirals directly target the proteins that HIV uses to replicate., while other antiretrovirals block HIV from invading host cells.
Even when taken together, these drugs can't cure HIV. As far as we know, a person infected with HIV will always be infected. However, if antiretrovirals are taken regularly, they can keep the virus at bay and turn what was once a death sentence into a treatable condition.