In a turn, if the airplane is moving sideways toward the low wing, it is said to be slipping. If it is moving sideways in the direction of the high wing it is said to be skidding. If it is moving with no sideways motion the turn is said to be coordinated. If you were sitting on board the aircraft, in coordinated flight, you would feel no side force, In a slip you would be pulled toward the low wing, and in a skid you would be pulled toward the high wing.

Pilots of full scale aircraft (unless they are doing aerobatics) try to fly coordinated because it's more comfortable for them and their passengers. It is also more efficient because there is less drag. R/C pilots usually don't care because they don't feel the side force, and it is hard to detect visually unless it is pretty extreme.