Basically a deadlift, but without beding the knees. It will work the hamstrings very deep.
Here's a good link with more information, and a small animation of a properly executed Stiff-Dead.
Barbell Stiff Leg Deadlift
Mt Snow, VT - May 5, 2012 - Completed!
Gunstock, NH - June 1, 2013 - Completed!
...i remain, he who remains to be...