A flock of geese in flight is called a skein of geese. When they are not in flight, a flock of geese is called a gaggle.
A skein of geese in a "V" formation is also called a wedge. Geese are not the only birds that travel in skeins; the term also refers to ducks and other migratory birds that travel in flocks.
Geese fly in a "V" formation to conserve energy by reducing wind resistance. The formation also helps the geese to keep track of every bird in the flock. By travelling in this manner, a skein of geese can fly further and for longer, as they do not tire as easily.