From A Wiki of Ice and Fire
A bravo is a type of young swordsman commonly found in the Free Cities who wishes to prove his martial skills. Bravos often go about in pairs. They practice and style of swordfighting known as water dance.
Bravos use slender swords that are edged and balanced for the thrust. They fight from a sideways stance with weapon extended.
It is said amongst the Braavosi that the night belongs to bravos and courtesans. When night falls, good citizens of Braavos retreat indoors, closing shutters and barring doors. Bravos swagger through Braavos in their parti-colored finery, looking to pick fights and prove their skills.
Most will fight with little cause or no cause. Anyone carrying a sword after dark in Braavos can be challenged to a duel. Simply placing a finger on the hilt of a sword while facing a bravo is akin to challenging them. Bravos never bother anyone without a sword. They are known to duel by the Moon Pool at night. Dying bravos often find their way to the House of Black and White to hasten their ends.