The web site is
Premier Pet. Great site, but unless it has changed in the last few weeks, I can't find anywhere to place an order, so I usually just call them at 888 640-8840.
A Gentle Leader looks something like a horse halter when it is in place. One strap goes around the nose & has the effect of a more dominant dog placing his mouth around the muzzle of a subordinate. The second strap fits (very snug) around the top of the neck at the base of the skull. Even though it is tight it does not choke the dog. (Try pressing on your own windpipe below and then above the adam's apple, and you can feel the difference.) My understanding is that this presses on an accupressure point that causes relaxation (the same spot the mother dog uses when she picks up a pup).
The net effect is to tell the dog "you're not in control of this situation, but relax, because someone else is in control".
They don't always work on every dog, but when they do, the effect can be dramatic and immediate. They seem to calm down aggressive dogs and give timid ones more confidence.
Make sure you get a Gentle Leader brand. There are other headcollars out there, such as the Halti, but they are not built the same way and in my opinion do not work nearly as well.