main characteristics Gothic: flying buttresses, windows with tracery, piers composed of colonettes or shafts bundled around a core. Skeletal system vs. mass of wall necessary in Romanesque architecture. Big windows vs small windows. emphasis on the VERTICAL.
One of the highest achievements of Italian Gothic art and architecture is Orvieto Cathedral. Few buildings in the world are that ornate, and at the same time - few have such a well balanced and harmonious architecture. A very good text about it can be read here.