Backfires in a vehicle happen for several reasons. Most are easily resolved with tuning, but some require the replacement of failing units. I will go over some of the major causes of this backfire below:
Backfire can often times be caused within our fuel tanks. The reason being that we don't keep our fuel tanks full at all times. In fact many of us, never fill up the tank at all. By having your fuel tank half full or less over a period of time, condensation is allowed to build up. This condensation, runs to the bottom of the fuel tank as it increases. As the engine demands more fuel, it is sucked up by the fuel pump and pushed into the fuel filter. As the water is heavier than the fuel, the water sinks to the bottom of the filter, where it builds up over time. After your filter fills up enough and you need to 'gun it', suddenly your vehicle needs a great deal more fuel and just sends the fuel along with all of the water into your carburetor or fuel injectors depending upon the age of your vehicle. This of course can create a backfire. The easy way to fix this is to just purchase water remover that you can add to your system. Another method is to just change our your fuel filters. I normally change my fuel filters every other oil change. I find that by doing this my vehicle runs much more efficiently. However, the use of water remover works just fine as well.
Another major cause of backfire is a write that could be shorting out in your electrical system or even a spark plug. If a spark plug misfires, this could easily create backfire in your exhaust. Regardless, these are relatively easy fixes and somewhat inexpensive. To best avoid spark plug issues in the future, I recommend changing out your spark plugs and replacing them with Iridium based ones. These tend to last substantially longer and are a lot more efficient than standard ones. They are quite a bit more expensive, but definitely worth the money overall.
Other than that, you can find that many performance parts can also cause your vehicle to backfire, especially if you haven't upgraded your fuel system while doing these upgrades. Exhaust upgrades can sometimes create air pockets in your system that once released create a loud backfiring sound. This isn't quite the same as a standard backfire, but sounds relatively similiar. This of course is completely harmless, and would require a lot more effort and custom work in order to resolve the issue.