Though many cooks freely substitute one for the other, each pan is designed with specific functions in mind.
A skillet's sloping side allows you to turn and remove food such as scrambled eggs more easily. In contrast, the comparatively high, vertical wall of a saute pan interferes with these cooking tasks. The rationale behind its construction is different: The design is meant to reduce the amount of oil that splatters beyond the saute pan's rim when, for instance, the cook pan-fries chicken.
The sides of a saute pan, incidentally, should not measure more than 2 1/2 inches. Higher walls cause excess steam to build up in the pan as gaseous water molecules are released by the frying foods. Moreover, some of the imprisoned steam molecules then con-dense and fall into the oil, needlessly causing extra splatter and lowering the oil's temperature at the same time.