The basics of Springs
The springs are suspension components whose job, on a road car, is to control the rate of compression of the shock absorbers. Soft springs means the shock absorbers compress faster, providing a smoother ride and keeping the wheels in contact with the road more often. It comes at the price of body roll and changing center of gravity, which makes the car feel like it's pitching and rolling through corners, decreasing the ability to change direction.
Harders springs are used in performance cars to reduce the body roll and increase the handling of the car.
In a Formula 1 car the springs are physcially attached a bit different but absorb the initial impact of bumps in the road. Simply put, the softer the springs the more grip are car can generate in the corners (it can corner faster).
Springs also play a part on ride height (which thanks to the undertray of the car, and the diffuser, generates as much as 40% of the overall downforce on a Formula 1 car).
Springs in F1 2011 don't perfectly mimic their realworld counterparts, perhaps in part because the game doesn't have adjustment settings for dampers and packers too.
If you're care is understeering into corners and you feel you have the setup otherwise right, try reducing the front spring to enable more front grip. Beware though, too much and your back end will be too loose and you'll lose it.
In F1 2011 pre Patch 2
Springs in F1 2011 before Patch 2 were very broken. 2 clicks harder would result in a consistent 2 second performance gain regardless of the circuit.
I'll start with same raw performance figures for springs. As a basis for the test I ran laps of the Albert Park circuit in Melbourne Australia for the middle Quick Setup option, changing only the spring settings.
With the stock settings at 6/6 the following times were recorded
With all else left the same the springs were adjusted to 8/8
I also peformed some runs with other settings.
4 Front - 4 Rear : The car was consistently 2 seconds per lap slower than with the 6/6 setting. The handling this time wsa noticeably worse in comparison to the 6/6 and 8/8 setup, both of which felt similar.
2 Front - 2 Rear : The car was basically undriveable and a competitive lap time was not able to be set. There were two noticeable characteristics here. When braking heavily the front tyres would easily lock and would lock heavily. On corner exit the car would oversteer massively (the back would come round and I'd be spinning).
8 Front - 2 Rear : This configuration removed the brake lockups but the car oversteered at a whim on once past the apex, using the same amount of throttle as a 6/6 or 8/8 configuration. Not competitive.
2 Front - 8 Rear : This configuration resolved a lot of the oversteer issues but left the car with massive brake lockups under anything resembling a heavy brake load.
In both the biased (front bias or rear bias) configurations the car handling was terribly imbalanced. This doens't necessarily mean that those spring settings should not be used at all.
Oversteer can be caused by a number of factors but the fundamental reason for it is because the front of the car is generating more grip than the rear of the car. To compensate for the behaviour seen by a spring setting such as 8/2 more rear wing can be added, or front wing can be taken away. Another possible cause of the oversteer is body roll in the corners. This can be remediated by increasing the rear anti-roll bar.
What I find confusing is that a softer spring setting should result in more rear grip and not less. It may be that corner entry speed is increased, as the car would be pitching forward and yawing, as it slowed for the corner, then the center of gravit would be pushed backward as the car soft rear compresses as the car accelerates. This should give more rear traction out of the corner too, as the weight would be over the back wheels more, and more weight equals more friction equals more grip (see What is grip and how is it managed).
Equally confusing is the behaviour of the car with a rear bias spring setting. With a softer front, the weight of the car should more quickly transfer to the front under brakes causing the car to understeer but definitely not lockup the brakes. Brakes are more likely to lock when wheels have less grip with the road. A wheel bouncing over bumps while braking will lock because it leaves the ground and has 0 grip.
Frankly, it seems the spring settings with regards to the dynamics of the car when springs are biased one way or the other are back to front.