Note: this site last updated in 2006
An article from "The Molecules of HIV" (c) Dan Stowell
gp41 is the bit of the envelope protein which sits within the virus particle. In fact, it is half embedded in the virus membrane, which enables it to bind with gp120 on the far side of the membrane.
The part of the gp41 protein which protrudes into the virus particle (the part that isn't embedded in the membrane) can interact with other gp41 proteins, linking together.
When a HIV particle binds to a cell, the interaction of gp120 with CD4 changes gp120's shape. This in turn changes the shape of gp41, in such a way that the gp41 helps to initiate the process of membrane fusion between virus and cell.