Depicted here and often seen in heraldry, it's a two-legged winged dragon with a barbed tail
They are winged and frequently have a barbed tail (
sometimes ... depicted as dragon-like critters with only one pair of legs instead of 
two (or ... Wyverns have two smallish wings and only have two short legs (that .... 
Wyrms are often depicted as being more serpent-like than Wyverns.