An expedition ship exists to get you off it. Everything about it is built around that: a mudroom, a fleet of Zodiacs, a large guide team, an ice-strengthened hull, and a daily plan the expedition leader rewrites based on ice, weather and where the wildlife is. A conventional cruise ship is built around staying aboard, with the ports as punctuation. In Antarctica the ship is transport and base camp, not the destination.
The practical differences you'll notice: no fixed itinerary you can hold anyone to, two off-ship excursions a day rather than shore days, an evening recap instead of a show, and a wardrobe that's mostly waterproofs.
The other thing that surprises people is the pace. It's roughly 7am to 9pm and it's an expedition rather than a holiday. Worth knowing before you book if you were picturing a restful week.
Within expedition ships there's a further split between expedition-leaning and luxury-leaning vessels. Both land you in the same places. They differ on what the hours between landings feel like.