Dragonflies and damselflies are two related groups (suborders) that make up the insect order Odonata. Although very similar in many ways, they are also very easy to distinguish.
Dragonfly adults have a rather robust body, and perch with wings held out to the side. Their eyes are huge, often meeting at the top of the head. The bodies of damselfly adults are quite slender, and for most species the wings are folded together over their abdomen while at rest. While their eyes are very large, they are set somewhat to the side of the head rather than dominating the front.
Dragonflies are swift and strong fliers, reminiscent of tiny airplanes, while damselflies have a rather fluttering flight.