These two aren't settings, but rather the result of all of the steering and suspension chassis choices made. Understeer, also known as plow or push, is the tendency of a vehicle to resist making a turn and "push" toward the outside of the curve. What's actually happening is the slip angle is greater at the front than the rear, due to alignment settings, center of gravity, soft suspension, aerodynamics, or a combination of these and other influences. Since understeer is inherently easier to predict and control, and prevents pushing a car to its traction limit, typically new car manufacturers err toward understeer on factory alignment settings in an effort to keep unskilled drivers safe. In racing circles where chassis are dialed-in for lap times, however, understeer is typically used to refer to the inability to follow the desired line through a corner when a vehicle has hit its traction limit.
Oversteer is the exact opposite; the slip angle is greater at the rear of the car due to any number of the same influences found in understeer, so it tends to get "tail happy," or rotate toward the outside of a turn and will usually result in a spinout without counteractive steering. Cars that exhibit a tendency to oversteer are significantly less stable at any speed, but the effects are exacerbated by driving near the car's limits. Typically, track cars are set up with a slight amount of understeer for stability, but some drivers prefer the quick rotation available with a touch of oversteer-especially drifters.
This is probably the most basic concept that many people overlook when upgrading muscle cars. Most classic cars have steering wheel diameters 15 inches or larger because a larger wheel offers more mechanical advantage, which makes steering easier, especially in cars without power assist. Nevertheless, it also works to decrease the effective steering ratio. Simply by dropping the steering wheel diameter, say from 15 to 13 inches with a custom steering wheel, the steering response will be noticeably quicker. It'll also be noticeably harder to turn at slow speeds, necessitating either power or "Armstrong" steering, but the benefit can be almost as transforming as changing the steering box ratio.
The nomenclature is a little misleading here since slip angle doesn't pertain to actually slipping or sliding of the vehicle. What it's referring to is the deformation of the tire under load, resulting in a difference of angle between the contact patch of the tire relative to the angle the wheel is steered. You're more familiar with this than you might think; imagine a standard sedan with 15-inch wheels and 70-series tires taking a fast, sharp turn. The tire's carcass and tread rolls and deforms, changing the tire's contact patch causing loss of traction. We actually found a YouTube video (www.YouTube.com/watch?v=W8UiE7yvO_M.) that offers an excellent example.
As a general rule of thumb, low-profile tires offer much less sidewall deformation, keeping angles low, while high-profile tires can go deep into the double digits. Notwithstanding, beyond the tire's ability to resist deformation, weight transfer in a turn from the inside to the outside tires (as illustrated in the video) is the biggest culprit. The more speed a car is capable of carrying into a turn, the larger the lateral forces acting against the outside tire. More equally loaded tires resulting from a well-sorted suspension will limit weight transfer, and will run at smaller slip angles. Of course, larger disparities will cause the slip angle to increase.
It can actually get much more complicated than that, since slip angle has a direct impact on Ackerman geometry, but as far as steering is concerned, the main consideration is working to create front-to-rear slip angle ratios that are as close to 1:1 as possible. Ratios above 1:1 tend to create understeer, while ratios below 1:1 result in oversteer. Eliminating slip angles is impossible, but controlling them will create more balanced handling and steering response by proxy.