Fresco is a windowing system derived from a structured graphics toolkit originally based on InterViews.
Fresco is designed to replace the XwindowSystem. It can run on top of X, or standalone.
As far as I can tell, the major difference between Fresco and the X Window system is that Fresco uses vector primitives (lines, characters, splines, etc.) to give it resolution-independence, while the X window system is pixel-oriented. I'm quite sure this is not "the major difference", in part because I used to know a fair amount about Interviews (see below); googling could come up with a better description.Fresco does have pixel-oriented extensions, and I've been told that the X Window system has some vector-oriented extensions (Render).
Fresco (FrescoFramework is just a WikiName) was/is the successor to InterViews. It brought CommonObjectRequestBrokerArchitecture (CORBA) to the GUI table. Fresco had a bumpy history. It was abandoned around '98 due to funding problems (Did Fujitsu fund this?). It had been resurrected by TheBerlinProject in 2002. TheBerlinProject even renamed themselves to Fresco.
MarkLinton was the primary designer of Fresco.