An ordinary boat has a V-shaped hull. It is built to cut through water, which it does well, and to lean and roll while doing it, which is less welcome when you are holding a drink.
A pontoon sits on two sealed aluminium tubes with a flat deck bridging them. The width between the tubes is what makes it stable — it resists rolling the way a table resists tipping. At anchor it stays put almost completely, and underway it stays level rather than leaning into turns.
The second difference is the deck. Because the tubes carry the buoyancy, the whole area on top is usable and flat, end to end, with no hull shape intruding. On a boat of the same length you would be sitting in a moulded cockpit. On a pontoon you can stand up and walk around.