Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
CXCR4 (aka "fusin") and CCR5 are both chemokine receptors. That is, they are proteins normally embedded in the membrane of a cell, and responds to the presence of chemokines outside the cell to trigger some kind of response within the cell.
HIV-1 is able to use either CXCR4 or CCR5 as a co-receptor (CD4 being the main receptor) to facilitate binding and entry into T cells. There are other possible co-receptors, but these are most commonly used.
CXCR4 was the first HIV coreceptor to be discovered, and it's one of the most important ones.