There are total of 6 parts constituting the cars. Each parts improve the different facets of the car performance
Brakes is related to how fast the car can decelerate, thus high brake performance relates to longer high speed retained before the brakes have to engage to prepare for the corner.
Engine is related to the top speed of the car. It is not clear if it relates to the car acceleration.
Front Wing is used for slow corners. The more front wing you have the faster your car will go around slow corners.
Gearbox relates to the acceleration of the car. It is important for the maps involving multiple corners.
Rear Wing is used for fast corners. The more rear wing you have the faster you can go around fast corners.
Suspension relates to the maximum speed of the car going around the corner. Higher suspension stat is useful for outside overtake around the corner.
Base Stat Improvement Edit
When a season has ended the parts that were in the car at this time become your new seasons base parts. This is only counting for the performance, but not for the durability of the car's parts.