Weapons increase your vehicle's overall power. There are several grades of weapons. The different weapon grades are Common, Uncommon, Superior, Epic, and Legendary. Higher grade weaponry will do more damage than their equivalent more common counterpart. Weapons come in four types, Hood, Side, Roof, and Front. Each has a specific job that they excel at.
Hood: Includes guns, lasers and other long range weaponry. Hood mounted weapons can only shoot straight forward which means that you have to aim in order to hit your target which requires more precision than the other weapon types. These can be really good at killing zombies before you run them over meaning less interference when driving. A permanent boost can be bought in the store, making you have dual hood mounted guns.
Side: Mostly saws, blades and other slicing weapons made for carving through zombies that you miss when driving past them and for taking out zombies that climb onto the car. Runners are less trouble if you have a side-mounted weapon since they can't stick to your car for long.
Roof: Consist of homing missiles, rocket launchers and other missile-based weapons made for killing zombies without worrying about aiming. This is really good at killing multiple zombies with a barrage of missiles or to kill some of those stubborn bosses. Even the fastest reloading roof weapons are somewhat slow but make up for it with higher damage than the other weapon types.
Front:Mostly barbed wire and spikes, these are made to smoothly run over zombies and take out anything directly in front of you. A fully upgraded superior weapon in this slot (obtainable from the boss of chapter 1) can easily take down bosses in a few hits. They are a good weapon to have due to not having a reload time. During missions you can find powerups that quadruple your damage or make them fire faster.
Beware, cars have slots meaning that they can only equip weapons that the car has slots for. Most cars have few slots so choose your vehicle wisely. Generally the more rare a vehicle is the more slots one will have however there are exceptions.