Having been to Gardone several times and having talked with top engravers like Firmo Fracassi and Luca Casari, I got the impression from them that engraving is defined according to the tool used to make it. Bulino is fine engraving done by hand pushing a graver. Intaglio is cut engarving using a hammer and special chisel. Then comes cesello which is sculpted engraving with near three dimensional designs on the steel.
The style is another matter. Each gunmaking area reflects the general culture of its area, so central European is different to English and Italian styles and so on.
Then there is the personal style developed by top engravers, like Fracassi's bulino rendition, so fine it almost looks like photography.
Somewhere in the 1980s engraving became a separate art and a bit of a fetish. Before that it was mostly routine "house" styles repeated by hand on each gun. They call this craftsman engraving to distinguish it from artistic engraving. There is correspondence on record (by Dicksons if I recall) where three guns were sent to the engraver on Thursday and were expected to be fully covered in the "house" style and be ready by the following Tuesday. Craftsmen apparently worked fast and at a reaasonable price.
For more on art in shotguns see here:
http://oplognosia.com/?p=9471