Dragonflies are insects in the sub-order Anisoptera (meaning “unequal-winged”). In Great Britain and Ireland there are about 30 species that may be encountered, whilst Damselflies are insects in the sub-order Zygoptera (meaning paired-wings). In Great Britain and Ireland there are over 20 species.
The term ‘dragonflies’ is sometimes used for the whole scientific order Odonata that also includes the sub-order Zygoptera (damselflies). British Dragonfly Society