A martingale collar is basically a collar that tightens to a certain point. They're great for dogs with narrow heads, and dogs that can slip normal collars.
Here's a good link to what a martingale is, that one looks like, and how they work:
http://www.collargirl.com/how_martingale_work.htm
I don't believe maxfox has a website yet, but you can always PM her (maxfox) with any questions.