Dragonflies are the world's largest and fastest insects. There are more than 6000 species known all over the world. (For the statistics: there are only about 4000 species of mammals known!) The insect order Dragonflies (or Odonata) is divided in three suborders: the "real" dragonflies or Anisoptera, damselflies or Zygoptera and a small third suborder, containing only one Japanese and one Nepalese species: the ancient dragonflies or Anisozygoptera.

Real dragonflies or Anisoptera are generally larger than damselflies. At rest they spread their wings alongside their body. The shape of the forewings is different from that of the hind wings: the hind wings are wider at the base and have a larger surface (Anisoptera means "unequal wings"). Their eyes touch on top of their head or the distance between their eyes is smaller than the width of their eyes.
The smallest European dragonfly is Sympetrum danae (Black Darter), which is only 32 mm. The largest species in Europe is Cordulegaster heros, of which the female can be as long as 97 mm.

Stout body, wings spread in rest: Dragonfly (Cordulegaster boltonii, male)

Damselflies or Zygoptera keep their wings above their back when resting. The hind wings have the same shape as the forewings (Zygoptera means "equal wings"). The distance between their eyes is larger than the width of the eyes themselves. Their body is slimmer and usually shorter than that of dragonflies. The smallest European damselfly, Nehalennia speciosa, is only 23 mm long, whereas the largest damselfly in Europe is Calopteryx virgo (Beautiful Demoiselle), which can be as tall as 49 mm.

Slim body, wings above back in rest: Damselfly (Enallagma cyathigerum, male)

The ancient dragonflies or Anisozygoptera seem to be a combination of dragon- and damselflies: they have a fierce, heavy body and eyes that touch, but keep their (Zygopteran shaped) wings above their back when resting. No species of this suborder are found in Europe, in fact it only contains two Asian species.

In Europe, there are 128 species of Odonata (take a look at the list!), 45 of which are Zygoptera and 83 Anisoptera. There are also 25 subspecies of Zygoptera and 26 Anisoptera subspecies. In The Netherlands, where I live, only 70 of 128 European Odonata were found. Nowadays, it's thought, The Netherlands have about 60 species.