A creature with a fly speed can move through the air at the indicated speed if carrying no more than a light load; see Carrying Capacity, page 161 of the Player's Handbook. (Note that medium armor does not necessarily constitute a medium load.) All fly speeds include a parenthetical note indicating maneuverability, as follows:
Perfect: The creature can perform almost any aerial maneuver it wishes. It moves through the air as well as a human moves over smooth ground.
Good: The creature is very agile in the air (like a housefly or a hummingbird), but cannot change direction as readily as those with perfect maneuverability.
Average: The creature can fly as adroitly as a small bird.
Poor: The creature flies as well as a very large bird.
Clumsy: The creature can barely maneuver at all.
A creature that flies can make dive attacks. A dive attack works just like a charge, but the diving creature must move a minimum of 30 feet and descend at least 10 feet. It can make only claw or talon attacks, but these deal double damage. A creature can use the run action while flying, provided it flies in a straight line.
For more information, see Tactical Aerial Movement, page 20 of the Dungeon Master's Guide.