Brawls are a game wide feature that is available to players from the moment they join a faction.
The idea of city brawls is pretty simple. Thoughout the game there are several different cities that are come available (or unavailable) as you level. Factions have the ability to take control of various parts of one of many different cities. These different areas can range from all types of different situations like a hospital that lowers your cost on healing, stores that lower prices for members from a specific store or revenue generating buildings and even different quest based objectives though out the day, week or month that offer prizes to the gang who holds them the longest.
A gang can choose to send out as many or as few of their members out on city brawls (within the limits of the number of brawls they can start within a day). Also some cities have level restrictions on them that allow or deny players into these cities. This means it can be beneficial to a factions to have lower leveled new members as much as it is important to have very high level members in their faction.
When a faction attacks a city objective (lets say city hall) first what happens is a check to determin if any other factions have this objective. If there is then a battle between the two factions members (attacking and defending) will now battle. Members do not need to be alive while these battles occure and do not raise or lose your win / loss ratios. Defenders will always have a defensive bonus to their members and some cities may even offer offensive or defensive bonuses to their members so learn to know as much as you can about your cities!
If the faction wins they now control this objective and will start to reveive the bonuses they offer. These different objectives and have have massive affects on players, their stats, battles or game play in general. Try to work with your faction members who are in charge of gang brawls to find out which objects will best benefit you and your current goals within the game.
Factions will also only be allowed to attack and or move members to different objectives so careful planning will be needed when making your decisions on who to attack.

