The two tools are similar but they have opposite layouts. On a jointer the cutting head is on the bottom of the tool surface where the planer has the cutting head on the top. A jointer is basically a hand planer that's upside down (and motorized). A planer will take a body that has one flat side and cut the other side to be flat like the other. This means the planer is designed to make a board thinner than it was and parallel. The jointer is designed to make a side of the board flat. In an ideal world you'd use a jointer, then a planer to make sure a board has two parallel sides.