In the books
In the A Song of Ice and Fire novels, the Iron Fleet is the elite naval force of the ironborn. It consists of around a hundred warships dipping 100 oars or more. The Iron Fleet is not the entirety of the ironbon naval forces, which are estimated to number well over 500 longships. However, most of these ships only dip 20 oars and are meant for coastal raiding, not pitched naval battles and ship-to-ship engagements. The Iron Fleet is a permanent military force held to belong to the Iron Islands as a whole, not individual captains or houses.
The Iron Fleet spearheads any major military action. Its job is to destroy or pin larger enemy ships, allowing smaller longships to land on the enemy shores. The Iron Fleet was comprehensively defeated in the Battle of Fair Isle during the Greyjoy Rebellion, when the Royal Fleet under Lord Stannis Baratheon and the Redwyne Fleet under Lord Paxter Redwyne trapped it in a pincer movement and destroyed it. This allowed the royalist forces to attack Pyke and end the war.
Since the end of the Greyjoy Rebellion, the Iron Fleet has been rebuilt to its former size. It is commanded by Victarion Greyjoy from his flagship, the Iron Victory.