The most common case is Ends Fold. Each end dancer walks in a semi-circle, toward the adjacent center dancer, and finishes facing the center dancer. They still turn around halfway (a full 180 degrees).
If the end dancers are not facing the same direction, the result will be a "Z" formation. For example, from a two-faced line after an Ends Fold the original end dancers will be facing the center dancers, but they will be standing by themselves, not holding hands with anybody.
If the end and center dancers were facing opposite directions, as in a wave, then after an Ends Fold the original ends will be facing the backs of the original centers.
Cross Fold works the same way except that the designated dancers go to face a spot in the other half of their line. For example, for an Ends Cross Fold, each end dancer walks in a semi-circle to face the other center position, rather than the adjacent center position.