The Arts of Thailand, by Steve Van Beek and Luca Invernizzi Tettoni, gives the following definition of chofa: "variously translated as 'bunch of sky' or 'tassel of sky', the slender finial like a stylized bird's head graces either end of a roof peak of a bot [the monks' congregation hall] or vihan [worship hall]. It is thought to signify the garuda and may originally have been intended to render Buddhism more appealing to Vishnuites, the garuda being Vishnu's mount." (p. 241)
In some buildings, the chofa "grasps two nagas [serpents] in its claws...", with the nagas sloping down the edge of the roof. The chofa has significant import ance, and some buildings are not considered consecrated until chofas have been fixed in place.
The naga is a snake, sometimes with many heads, and one particular naga (the serpent king Muchilinda) played an important part of the Buddha's path to enlightenment (photo of a sculpture). Ornate and fierce-looking snakes are on the sloping edge of many roofs in Thailand as part of the protection around the holy site. Visit Khandro.net for much more about nagas.
See also Glossary of Temple Terms, Temples, Thai Architecture, and Temple Talk at various Thai tourism sites.
(top and lower left photos: from Wat Phra Kaew compound in Bangkok; lower right photo: monastery complex near the Golden Mount, Bangkok)