In biological classification, a group of related families. For example, the horse, rhinoceros, and tapir families are grouped in the order Perissodactyla, the odd-toed ungulates, because they all have either one or three toes on each foot. The names of orders are not shown in italic (unlike genus and species names) and by convention they have the ending -formes in birds and fish -a in mammals, amphibians, reptiles, and other animals and -ales in fungi and plants. Related orders are grouped together in a class.
Source: Hutchinson Encyclopaedia