Dragonflies as a group make up about half of the order Odonata. This is an ancient order that in the fossil record goes back approximately 325 million years; to the Carboniferous period. Their ancestors the (Protodonata) reached incredible proportions in that we have found fossils of them measuring up to a foot (30 in.) in sheer wing expanse.
The entire order Odonata nowadays comprises a little over 6000+ species; with dragonflies alone making up about 3000 of that number (give or take). This order has an enthusiastic following and many books surprisingly have been written on them oftentimes featuring many lavish photographs.
In the insect world Dragonflies reign as the "Lords of the Air". There simply is nothing that can match them in power or more importantly -- agility. They can catch a fly, midge, or mosquito on the wing. Their compound eyes are without compare. I have read their eyes have nearly 24,000 ommatidia each! Those are the very tiny facets or individual lenses which are present.
My photograph below illustrates just some of the possible variety amongst the group. Dragonflies can reach fairly good sizes as seen in my darner dragonfly (Anax) from (Oahu, Hawaii). They may also be elegantly patterned on their wings or may delight us with their bold coloration.
Below, my first photograph is THE largest extant species which I know of: Petalura ingentissima from North Queensland Australia. This one regularly measures in at 7 inches in wingspan. The true "Emperor" if you will of the lot...