Original: Dragonflies are similar to damselflies, but the adults can be differentiated by the fact that the wings of most dragonflies are held away from, and perpendicular to, the body when at rest.
Simplified: When at rest, most damselflies hold their wings together above the body, whereas most dragonflies at rest hold their wings horizontally or occasionally slightly down and forward. The eyes of dragonflies are larger, and touch each other.