Like just about any animal classification, there's a bit of arbitrariness over where the lines are drawn at any given time. Between ducks and geese, the line is mostly about size. Geese are large - think Canadian Geese -
Ducks are small, like mallard ducks -
Ducks usually eat fish and insects, while geese usually eat grains and grasses. However there are always exceptions to those rules. If you collect down, goose down has less odor than duck down does. Geese tend to have more webbing between their toes than ducks do.
In fact, some people call geese "grey ducks". Which points out another difference - geese tend to be grey or white or grey-white with black. Ducks tend to be all sorts of interesting colors, from green to brown to spotted and so on. There are no native white ducks in North America. However, a common domestic duck is the Pekin Duck, which looks like Donald Duck of the cartoons.
So get to know the ducks and geese that are common in your area - and enjoy the wildlife!
