Max Hailstorm inflicts damage and creates hail for 5 turns (8 if an Icy Rock is held). Its power and whether it is a special or physical move depends on the move it is based on.
Dynamax Dragonite forms a big blue cloud over its head. Multiple huge balls of ice then fall down from the cloud and onto the opponent. When used, the move causes hail to fall over the battlefield.
This article is part of Project Moves and Abilities, a Bulbapedia project that aims to write comprehensive articles on two related aspects of the Pokémon games.