There are many different groups of hymenoptera (the insect order that also includes ants and bees) that are called wasps, including an awful lot of very obscure parasitoid wasps. However, when most people think of wasps they think of the black-and-yellow variety that build paper nests and live in colonies. These common wasps are classified in two, very closely related genera: dolichovespula and vespula (the common wasp is vespula vulgaris, for instance). Hornets (with one British species and several in Asia) are the closest relatives to these wasps but separated from the ancestors of dolichovespula and vespula long enough ago that we place them in a separate genus: vespa. As well as being substantially bigger than a wasp, the hornet can be separated from wasps in Britain by its ‘fat head’: looking from above, the head expands behind the eyes whereas in vespula and dolichovespula the head narrows behind the eyes.
Answered by Dr Gavin Broad, Entomology, NHM, London.