A very thick pizza: lots of thick crust, sauce in the middle, tons of cheese. Lots more details on the Wikipedia page.
Local Detroit-style pizzerias:
Made in blue-steel industrial utility pans that were originally intended to hold machine parts.
Resellers in Detroit include People's Restaurant Equipment.
A shortage in late 2010 - early 2011 was caused by the manufacturer shutting down its Michigan plant and moving to Mexico. Production had not resumed by early 2011, leaving suppliers short. Eugene Jett of Jet's Pizza reverse-engineered the pans and started manufacturing in Michigan to meet Jet's demand.