In the USFS competitions I've seen, Artistic generally has no props and looks more like a freeskate program without jumps. Emphasis is on expression of the music, timing, flow and presentation. The announcement describes the criteria and may vary from competition to competition. Showcase is more entertaiment style programs: costumes and props are encouraged and very often the application asks for the name or theme of the program. Characters are common. Sometimes jumps are limited but the judging criteria may include technical difficulty. Again read the announcement for the description.
In my mind, I think of Artistic as closer to Show Dance and Showcase as Entertainment Free Skating, but that's just my simplistic shortcut.
In ISI, I think Artistic is a form of Free Skating only without the jump requirements, but I could be wrong.