Scrub Radius
Scrub radius is basically the centerline of the wheel relative to the steering axis inclination (SAI). SAI is easiest to explain by breaking the term up. The steering axis is the line between the top pivot point of the spindle (the upper ball joint on cars with upper and lower control arms) and the lower ball joint. The inclination of the steering axis is the angle between the steering axis and the centerline of the wheel. Now to find the scrub radius, we follow the SAI all the way to the ground, and measure the distance between that point and the centerline of the tire patch. If the tire contact patch is outside of the SAI pivot, the scrub radius is positive. If it's inboard, it's negative. Unless you're swapping to a new style suspension system, or willing to do some complicated customizing, scrub radius is set at the factory and not adjustable.

So why do you care? Well, the point where the SAI meets the road is the pivot point on which the tire is turned, so scrub radius has a great deal to do with how the steering feels and how much feedback is generated through the steering wheel. Also, scrub radius is directly affected by wheel offset, so it's a good idea to keep that in mind when pondering new wheels. Small changes won't make much difference, but large ones can affect vehicle handling notably and change the steering dynamics. For example, wider wheels with minimal backspacing create positive scrub radius because the centerline of the tire patch is moved further outside the SAI, which places more stress on steering components and typically increases steering effort at low speeds. On the upshot, it also increases steering feedback and feel, which is vital for control. Conversely, negative scrub radius tends to decrease steering effort and feel.

Generally speaking, older muscle cars tend to have a longer scrub radius, and modern performance cars have a short scrub radius (usually only an inch or so). Sometimes cars having a longer scrub radius experience "tramming" on roads with worn surfaces, such as where heavy vehicles have created ruts. This can create a wandering feel that is sometimes confused with bumpsteer. Wide front tires can also exhibit negative effects of a longer scrub radius as the road camber changes, whereby the road acts alternately on the inside and outside of a tire.

Toe Angle
At its most basic, static toe angle is just the degree the front wheels deviate from parallel to the centerline of the vehicle. On the road, near-zero toe is ideal for tire wear, but due to the flex created by the numerous suspension bushings, compensation is necessary. To get close to parallel at speed, rear-wheel-drive cars always require a slight amount of toe-in, because the forward thrust from the rear wheels causes the compliant rubber bushings in the front suspension to flex rearward slightly. Front-wheel drive is exactly opposite, and requires toe-out, but for the same reasons. Cars upgraded with Heim joints rather than bushings experience far less flex.

There are other considerations as well; toe-in, meaning the centerline of the wheels, will converge at some point ahead of the car. This aids stability at speed and for self correcting, but too much toe-in causes accelerated wear at the outboard edges of the tires. Too much toe-out causes wear at the inboard edges and skittishness over bumps and grooves, but serious track cars often accept that trade-off for the increased steering response available from a light amount of toe-out. That brings up dynamic toe.

Ackerman geometry naturally produces toe-out on the inside wheel in a turn, which helps turn-in, but toe is actually a dynamic setting that varies according to the forces applied on the wheel, and according to camber gain. This concept is actually closely associated with roll steer, which is the result of one wheel rising as the other falls due to weight transfer and cornering force, as in a hard corner. The result is more toe-in on one wheel, and more toe-out on the other, consequently producing a steering effect.