Suspension stiffness
This obviously sets up how hard the car's springs are. In general these should be increased and decreased in proportion to each other, unless the car lacks an anti-roll bar at one or other end. an overall stiffer car will have more grip on smooth surfaces because it will roll less. it will lose grip on uneven surfaces, though as the tyres wuill find it harder to maintain contact.
Ride height (overall)
how high the car is from the road. or to put it another way, how much movement you want to allow the suspension to have. stiffer cars can run lower, and soft cars require a higher ride height to avoid grounding and hitting the bump-stops. really this setting and suspension stiffness go hand-in-hand.
Final Drive ratio
Basically this wants to be set as low as can be done without the car hitting the Rev Limiter in the top gear - this maximises the available acceleration.
Brake Bias
This affects the understeer/oversteer balance under braking. having this set towards the front will give you a stable car under braking which will tend to go in a straight line. moving this towards the rear will increase instability making the car easier to provoke into oversteer
Central Diff Torque Split
This essentially acts like Brake Bias, but for power- set this forwards for stability and understeer, and to the rear for power oversteer.
Downforce
The most fundamental tradeoff - straightline speed against speed in the turns. Low downforce will give you a fast car that won;t have much grip - high downforce gives you a grippy car that may be sluggish on straight sections.
Anti-Roll Bars (front / rear)
ARBs are the primary tool for tweaking the car's balance in the turns. They act like the main suspension stiffness, but only come into play when the left and right wheels at each end of the car want to be at different heights relative to the body.
As a general rule, a stiffer ARB at a given end will cause that end to have less grip- or rather will make it more likely that it will run out of grip first.
So a car with a soft front end, with a hard back end will tend to oversteer, and a hard front end with a soft back end will tend to understeer.
ARB settings are often used to correct the natural handling balance of different drivetrains bacuse they are a very powerful tool.
The soft front/hard back is common in Front-wheel-drive cars bacuse of the natural tenency they would have otherwise to understeer. you can see this in action if you look at pics of hot hatches and FWD touring cars being driven hard where you can see them "cocking a leg" and lifting the inside rear wheel in the air.
Similarly, powerful rear-wheel drive cars tend to run with a stiff front ARB and soft at the rear.