When most people talk of the virus HIV, they mean HIV-1. There are actually 2 distinct major strains of the virus, HIV-1 and 2. HIV-2 has effects like those of HIV-1, but is only found in certain areas. Very few (under 100) people are infected by it in the US, and those that do have been to where HIV-2 is most concentrated, the western portions of Africa. There are a few differences between the two strains. One is that HIV-2 is harder to spread, as it is only transmitted through sexual intercourse of from a woman to her child while it is in the womb, while HIV-1 can also be transmitted through these and through blood to blood contact and sharing needles. Another contrast is that those who are infected with the second form of HIV usually have a lower virus density in their bloodstream. This is the reason for the fact that the onset of AIDS is slower for those infected with HIV-2 rather than 1. A last difference is that most tests for HIV do not detect the second strain, which sometimes causes people in contact with areas prone to HIV-2 to ask for a second test designed just for finding it.
Oster, Dr. Harold. "HIV-1 & HIV-2." Family Health Resources. 15 Oct. 2000. iVillage. 14 Feb. 2004 <http://http://www.ivillagehealth.com/experts/infectious/qas/0,,242108_175312,00.html>.