First of damselflies are typically smaller and more slender then the thicker stout dragonflies, this is probably the reason for the name damselfly. Dragonflies are strong fliers with the ability for sustained flight while damsels have a much weaker fluttery style of flight.
The eyes of the damselfly are separated from each other, resting on opposite sides of their hammer shaped head, while the eyes of the dragon meet in the middle of their head, except the clubtails who have heads that are similar to damselflies. While at rest the damselfly will hold their wings vertically over their back except for the spreadwings which hold theirs at a 45 degree angle. Dragons always hold their wings perpendicular to their body while at rest.
One of the largest damselflies in size are the jewelwings. There are two types of jewelwings the ebony jewelwing and the river jewelwing. All of the pictures above are river jewelwings. You can tell them apart fairly easily because the ebony jewelwing has solid black wings where the river jewelwing has two-toned wings. The first three pictures here are of males, which have a metallic green color. Females, like the one in the last pic, are usually a yellow green color with a white dot on the edge of their wings.