Chytrids are, in the broadest sense, all members of the microscopic fungal Division Chytridiomycota, which contains one single class, the Chytridiomycetes. In narrower and older sense, "chytrids" are only those in Order Chytridiales. The article assumes the broader definition.
The chytrids are the most primitive of the fungi and are mostly saprobic (degrading chitin and keratin). Many chytrids are aquatic (mostly freshwater). There are approximately 1,000 chytrid species, in 1,000 genera, in 4 orders.
Both zoospores and gametes of the chytrids are mobile by their flagella, mostly just one whiplash per individual. The thalli are coenocytic and usually form no true mycelium (having rhizoid instead). Some species are unicellular.
Some chytrid species can kill frogs in large numbers by blocking the frogs' respiratory skins, as was first recognized in 1998 in Australia and Panama (see Decline in frog populations). There are also maize-attacking and alfalfa-attacking species.