Dragonflies are usually much bigger and thicker bodied than damselflies.
Both are very predacious in both the nymph and adult stage. Nymphs will feed on almost any creature in the lake including very small fish. Most of our Interior dragonflies are 'Darners' of the family Aeshnidae. I have seen a few 'Gomphus' dragonflies of the family Gomphinae and some Common Red Skimmers but not enough to include as a major component of the following discussion. Gomphus are also known as 'Clubtails' and the adults generally have a distinctive club-like shape at the end of the tail.