Colloquially, both "guppy" and "minnow" may refer to diminutive fish. In scientific terms, however, the words have fundamentally different denotations and refer to freshwater fish not closely related.
Taxonomy
The guppy is a single species of ray-finned fish, Poecilia reticulata. Minnows, by contrast, are a family of fish---the cyprinids---encompassing not just the tiny "minnows" of common perception but also goldfish and massive carp.
Morphology
Many minnows have distinctive barbels near their mouth and mobile upper jaws. While most are small, often under 5cm in length, large carp like Asia's giant barb may exceed 6 feet. Guppies, usually 2cm to 5cm long, are characterized by high-sided pectoral fins and a pelvic fin situated well forward on the body.
Geography
Guppies naturally inhabit a range of fresh waters in northeastern South America, a landmass with no native minnows. That absence notwithstanding, cyprids have the most extensive continuous range of any freshwater fish family, found on all other continents except Antarctica.
Ecology
Many minnows are bottom-feeders, serving important ecological roles near the bottom rungs of aquatic food chains. Guppies feast primarily on insects, though they'll also eat zooplankton and other food items.
Relationship with Humans
Guppies are one of the most popular aquarium fish in the world. Populations of Trinidadian guppies separated within a single river by series of waterfalls have been famously studied in research on natural selection. Minnows have immense importance to humans, as gamefish themselves or as bait and as a natural food source for other commercially desirable species.
Tags: freshwater fish, Many minnows