HIV stands for: Human Immunodeficiency Virus
HIV is a virus. Viruses such as HIV cannot grow or reproduce on their own, they need to infect the cells of a living organism in order to replicate (make new copies of themselves). The human immune system usually finds and kills viruses fairly quickly, but HIV attacks the immune system itself - the very thing that would normally get rid of a virus.
With around 2.6 million people becoming infected with HIV in 2009, there are now an estimated 33.3 million people around the world who are living with HIV, including millions who have developed AIDS.