Toe-in, the front of the wheels on the affected axle (rear) are closer together than the rear's of the same wheels (like a duck's feet). On the rear, this causes the car to increase traction (therefore less oversteer, more understeer), meaning more grip when accelerating. It also increases rolling resistance. Use a little toe-in on cars with too much oversteer.
Toe-out, this is the opposite of above, the front's of the wheels on the affected axle are further apart than the rears (opposite of a duck's feet LOL). On the rear this will cause the back end to step out more readily when turning in. The downside is that straight line stability is affected, because if one wheel loses grip (lock-up under braking fro instance), the other wheel will try to pull the back end out of line. Use toe-out for cars which have too much understeer.