A common misconception is that all pokemon that can fly are automatically Flying type, but in fact the Flying type is distinguished by aerial maneuverability and speed. While it is the third most common type of pokemon, the flying type is strictly secondary in nature and with the exclusion of one legendary pokemon always appears paired with another type. This makes defining flying types as difficult if not more so than defining the more common Normal type; still, a general rule of thumb is that if it uses flight to its advantage, it's flying type.
The majority of Flying type pokemon have evolved as insectivores and granivores, so they tend to be built toward destroying both Bug and Grass types with little resistance. Their three-dimensional manuvering also provides them a distinct advantage against fighting types; when they engage in combat, the flyer typically wears the fighter down by dodging its attacks and assaulting it from unplanned angles. The true strength of the flying type, however, is their immunity to Ground type attacks; for somewhat obvious reasons, ground attacks simply cannot hit a flyer unless they are roosting, and given the raw power that Ground attacks have this is considered a distinct advantage.
However, Flying types have very delicate physiology, generally having more nerve endings then none-flyers of their primary type due to the complexities of wings and sustained aerial maneuvering. For this reason, electric types are the bane of flyers, as they cannot stand their natural charge and are usually forced back from assault due to the pain. Ice types also hurt their wings (or other flight mechanisms) and rock type moves can take the wind out of a flyer's weak body. Steel type moves are, fortunately, slow enough to be partially dodged by flyers.
Aside from pokemon whose primary type is the Bug type, Flying-type pokemon tend toward avian body structure for obvious reasons. There are, however, exceptions to the rule, usually in cases of Legendary or Dragon type pokemon. Bug type pokemon in this class resemble bugs. Flying type pokemon can be found in every type class and in fact are the dominant hybrids of Normal type pokemon.
As previously stated, flying type pokemon are usually insectivores or granivores. This is not a hard and fast rule, however; flyer diets are usually tailored to their home environment.
It is not unusual for fliers to perform aerial dances when seeking mates.