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The different ways a tire can be unaligned
(sh.itjust.works)
sorry for bad alt text, I lack the terminology to describe this accurately
(i'm not a mechanic, i have no earthly idea if this is accurate. Don't sue me)
What's going on with the caster picture? Not sure what it is showing.
it has to do with the angle between the point the tire pivots (steers) on and the point the wheel spins on. The steering point needs to be forward of the spinning point to help the car go straight down the road. Think of a shopping carts front wheels (they're called casters), and how they'll pivot around with the direction you're pushing the cart. That angle can be adjusted so the car tracks properly...
It's showing the 'knuckle' the wheel is mounted to leaning forward and backward.
In addition to what others have said, it also affects camber in turns. Positive caster when turning gives the outside wheel negative camber and the inside wheel positive camber to keep a larger patch of contact when turning at high speed.