A jointer will create a perfectly flat surface.
Jointers have a spinning blade within the table.
Jointers cannot effectively shave a board to uniform thickness.
The planer will create a uniform thickness.
Planers have a flat deck and a blade that spins above the table.
Planers cannot make a board with perfectly square sides on its own.
They're not crucial tools for every workshop, but they definitely come in handy sometimes! As my woodworking skills improve, I find that I rely on these tools more often for high quality results.