In short, Dynasties are Families (or Houses) much like Lannisters and Starks in GoT, but Dynasties are less of a Faction and more of a Mini Region. you see, Arramar is ruled by the King which is currently the Harkinshire Dynasty. but there are many smaller Sub-Realms which are ruled by a Dynasty, the ruling Dynasty decides on everything that a King would do. but they don't have the right to have a large army (80K Max), they cannot attack other Realms without the King's Rule. but most importantly, they must follow the King to War if he calls the Banners.
List of Major Dynasties in Arramar
Trait: OCEAN SHIPS
Animal: Sea Serpent