Global

• Traction Control - Limits wheel spin under throttle. Less - allows the tires to lose grip more frequently.
  
• ABS - Allows the tire to rotate under hard breaking. Full + will simulate you pressing/depressing the brake pedal. Makes it so you can somewhat steer while fully on the brakes.
  
• ESP - Reduces the cars power in order to gain traction. Think of it like traction control but at the ECU level. More + will limit power on the car in order to maintain traction. End feel is car is slower in corners but less likely to slide.
  
• Slide Help - Counter-steering in drifting. More + helps with maintaining a slide and transitioning into a new one. With less you can transition drifts much faster but greater chance is a 180° spin.

Specific

• Aero Distribution - Side of the car airflow is pushing down. Different uses here. Less in the front will cause understeer.
  
• Diff Distrib - AWD Cars Which side of the car will have the most power. More + to the rear allows for oversteering.
  
• Gearbox - Gear Ratio. More + will give you more top speed but reduced acceleration. Less - Will have you slapping through gears quicker but reduces top speed.
  
• Tire Grip Front/Rear - Sticky Tires. Play around with these sliders. Reduced front produces understeer. Reduced rear produces oversteer.
  
• Brake Power - Pressure of the pads on the rotor. More + will slow the car down faster but also has a higher chance of locking up the brakes.
  
• Brake Balance - Ratio of front to rear pad pressure. More to the front will cause understeer during braking.
  
• Susp Comp Front/Rear - Upwards travel of the wheel over a bump. More + is stiffer. Higher it is the more likely you will lose initial traction over a bump. Good for TC where there are few bumps.
  
• Susp Reb Front/Rear - Downwards travel the wheel after a bump. Less - will cause the vehicle to be floaty. More + will make the car settle faster.
  
• Susp Geom Camber Front/Rear - Vertical alignment of the wheel to the body of the vehicle. Less - results in the top of the wheel more towards the body. More + is the opposite, bottom of the wheel towards the car. Generally, a little less will give you more traction in the corners but less on the straights.
  
• ARB Front/Rear - Anti-Roll Bar. More + allows less body roll. This stiffens the car. Less - will allow the weight transfer but also reduces the reaction to the turn. These can also be used to induce over/under steer.

Global

• Drift Assist - TBD

Specific

• Weight Distribution - Front to Back leaning. Acts very much like holding forward or backwards on the stick. More + is weight on the front. Reduces top speed but gains control.
  
• Rudder Angle - Surface area of the rudder in a turn. More + will allow you to turn much faster at the cost of speed.
  
• Braking Power - How quickly you can slow down. More + allows you to stop faster.
  
• Buoyancy - How floaty you are. More + will give the boat more surface area above the water.
  
• Hull Friction - Resistance of the water to the boat. Less - allows for some water drifting.
  
• Trim Tabs - More + allows quicker transition between left and right turns but at reduced speed.

Global

• Auto Climb Control - When set to On, it will maintain your altitude. When set to Off, you will start dropping altitude.
  
• Knife Flight Assist - While in a knife flight, it will counter control the rudder to help maintain your level.
  
• Automatic Landing Gear - Left on, gear will come out on its own near the ground. Pro mode will have it off, allows for more speed closer to the ground if you do not intend to land.
  
• Ground Cushion - Rate at which plane pitches up when near the ground. Less - reduces the distance to the ground.
  
• Ground Effect - The lift produced by real-world phenomena when flying close to the ground. Less - reduces this already slight effect.

Specific

• Plane Reactivity - Sensitivity of the Control Stick. Less - reduces how quickly the plane adjusts to your control. More + makes it snappy.
  
• Stall - Threshold at which you lose control. Reducing gains speeds but at the cost of possibly not being able to control your plane. It is suggested to reduce this to a setting at which you can still performs tricks or turns. Increase if stalling.
  
• Wings Dihedral Angle - How upright/vertical the wings are to the fuselage. (Note: There is seemingly barel to no visual difference in game, but the stats indicate that more + gives you better roll rates)
  
• Yaw Briskness - Rudder effectiveness. Less - reduces how much your plane will "drift".
  
• Pitch Briskness - The up/down efficiency of the African Swallow. Less - Reduces the rate at which you fall and rise.
  
• Roll Briskness - Do a barrel roll, but slower. - Less - slows down how fast you roll the plane.
  
• Stabilizer Incidence - Rear wing/stabilizer perpendicularness to the rest of the plane. More + will cause your plane to constantly reduce altitude.
  
• Wings Incidence - More + will cause your plane to constantly increase altitude.

Special thanks goes out to Xionergy who has originally posted this guide over on reddit and who has given me the permission to repost it on Steam.
Further thanks go out to miljon3 and zhpete who contributed to the original guide on Reddit and delivered further explanations about some of the settings.