A human immunodeficiency virus (HIV) test detects antibodies to HIV or the genetic material (DNA or RNA) of HIV in the blood or another type of sample.
This determines whether an HIV infection is present (HIV-positive). HIV infects white blood cells called CD4+ cells, which are part of the body’s immune system that help fight infections.

HIV causes acquired immunodeficiency syndrome (AIDS), a long-term chronic disease that cannot be cured.
Several tests can find antibodies or genetic material (RNA) to the HIV virus.

Read more: http://www.webmd.com/hiv-aids/human-immunodeficiency-virus-hiv-test