Flyaway wedding veils are a short wedding veil style with multiple tiers. The length of flyaway wedding veils just brushes the shoulders, and as the name suggests, the tulle used in a flyaway veil typically has a stiffer quality than other styles of wedding veils, allowing the flyaway veil to "fly away" from the wearer, adding volume and a little bit of "poof".
The flyaway veil is typically considered a more informal wedding veil style, most commonly seen paired with an informal wedding dress. (For more on the differences between formal and informal wedding dresses see: Formal Wedding Dresses vs. Informal Wedding Dresses).
Of course, these days, there are no strict rules governing a bride's style, and if she wants to have a formal wedding and don a flyaway veil at the same time, she certainly may do whatever suits her fancy!