When Forza Motorsport was released for the Xbox in 2005, it showed that Polyphony Digital were not the only ones that could make a great racing sim. It offered one of the best driving models to be seen at the time, along with a few unique features.
As of 2015, the series has received 5 mainline installments (Forza Motorsport, Forza Motorsport 2, Forza Motorsport 3, Forza Motorsport 4, and Forza Motorsport 5) and 2 spin-offs titles (Forza Horizon, Forza Horizon 2), with a 6th installment Forza Motorsport 6 releasing in 2015.
Forza Motorsport brought realistic car deformation to the sim racing genre. Up until that point, you could drive as recklessly as you'd like and there really was no penalty for doing so. Forza offered three levels of damage, cosmetic only, normal, and Simulation. On Simulation, you could do enough damage to your car to make it practically undrivable.
Forza Motorsport featured excellent opponent AI that was aware of where you were on the track. The developers took this a step further and introduced the Drivatar feature where you could train the AI to mimic your driving style. The result was an AI driver that had your strengths and weaknesses, but drove a little too consistently. This feature was later dropped in Forza 2.
Other games had allowed you to customize your car, but Forza took this to a whole new level. The Livery editor allowed you to use various shapes of decals to create your own unique designs. With a lot of time and some talent, you could make some truly spctacular car art. There were entire communities created around the game's Livery Editor, many of which are still going strong today.
Forza Motorsport allowed you to take your custom cars online and race against the world over Xbox Live. Up to eight players could race in a variety of game modes. The online component was expanded on for Forza Motorsport 2 to include tournaments and an Auction House.
The game allowed you to trade cars with other people via Xbox Live. Initially you had to set up a special trading session with someone else in order to get their cars. Forza 2 streamlined the process with its eBay style Auction House.
The Auction House debuted in Forza Motorsport 2 and was the much needed solution for people who wanted to sell their custom painted cars online. Instead of having to negotiate a one to one transaction, now you could simply list your car, set your minimum price and optionally set a BuyOut price. The Auction House has become the hotbed for some of the finest car Liveries around. There are also a few cars that can only be found via the Auction House, using the Livery Editor many Brilliant Paint Designs can be found thanks to the Forza 2 Auction House.
Tuning up your cars for best performance is almost a black art. Forza's tuning options let you adjust just about everything on the car, given you had the proper parts installed. Some people made quite a nice amount of in-game money by tuning other player's cars.