When Turn 10 Studios' 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.
Beginning in 2012, the franchise was divided between two series, with Playground Games' Forza Horizon (initially a spin-off of Forza Motorsport) having become a regular series in its own. There are currently seven Motorsport installments (Forza Motorsport, Forza Motorsport 2, Forza Motorsport 3, Forza Motorsport 4, Forza Motorsport 5, Forza Motorsport 6, and Forza Motorsport 7) and four Horizon installments (Forza Horizon, Forza Horizon 2, Forza Horizon 3, and Forza Horizon 4). Both series have also received spin-offs of their own; Horizon 2 received a standalone expansion to promote the Fast & Furious franchise's seventh film, and Motorsport 6 received a free-to-play release on PC known as Apex.
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 Motorsport 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 spectacular 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 Motorsport 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 buy-out 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 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.
Forza Horizon is the open world Forza series that uses many of the features found in Forza Motorsport. The games center around the fictitious Horizon Festival, a massive celebration of cars and music. Players complete events around the world, unlocking new events and cars as they progress. Forza Horizon is less of a racing simulation than Motosport, but it still allows players to upgrade and tune cars. The series also includes the livery editor.
Beginning with Forza Horizon 3, players can set their own rules for in-game races, such as car restrictions, time of day, laps, and weather, and share these "Blueprints" with others. This feature was expanded in Forza Horizon 4 to allow players to plot out their own courses.