Basically, dragonflies rest with their wings open, and damselflies rest with their wings closed. (Unless they're spread-winged damselflies, in which case the wings are partially open.)
Male Ebony Jewelwing Damselfly (Calopteryx maculata).
Immature male White-tailed Skimmer (Plathemis lydia).
Thanks BugGuide.net and Gloria Mundi Press for the ID help.