QK dies are ultra-thin dies. Some of them (the line is called "Cookie Cutters") are similar to Nestabilities. However, they do not emboss like Nesties and do not nest as evenly as Nesties do, IMHO.
In general, QK dies come in 2"x2", 4"x4" and new 12-inch-long border dies.
The 2x2 dies can be cut with a tool called the Squeeze. Then the 2x2, 4x4 and 12-inch longs can all be cut with a Revolution. (There is a 12" platform available for sale separately for cutting the 12-inch border dies.)
QK dies CAN also be used in other machines. I have a Big Shot from Sizzix and use my QK dies in there quite nicely.
http://www.quickutz.com for more info.