HIV is a virus. Its full name is Human Immuno-deficiency virus. As its name suggests, the HIV virus is found only in humans, and it attacks the immune system making you more vulnerable to infection and illness.

The last stage of the HIV virus is AIDS, or acquired immune deficiency syndrome. It's called 'acquired' because it's not genetic but you have to get it or catch it, and a 'syndrome' because it's not a straightforward illness with specific symptoms but can affect people very differently.

In order to develop AIDS, you have to catch the HIV virus first, but people have managed to live with HIV a long time (10 years or more is typical) without reaching the AIDS stage. Also, while there isn't yet a reliable cure for full-blown AIDS, there are drugs that slow the progress of HIV and make it possible for infected people to live longer and healthier lives.

You can learn more about the differences here.