All models capable of flight have the special rule Fly on their army list entry they belong to. For me sake of convenience they are termed as flyers. Flyers are moved during the player's Move phase along with other models.
There are two types of flyers: flying monsters and flying units. They share the same rules apart from a few exceptions given later. All flyer models are on square bases like other troops in Warhammer. This is more convenient and makes it easier to resolve close combats between flyers and other units.