A cluster on the other hand actually is a group of stars that formed from the same material and are thus very close to each other.
So there is a difference, but it's only a problem to interchange the terms the other way round.
A cluster could be treated as a constellation, but not vice versa.
For example, the Pleiades has a core radius of only about 8 light years, and almost all of the 1,000+ stars in the cluster are within a volume of radius 43 light years.
A constellation is a two-dimensional region of the sky, more like counties or states on a map. The "celestial sphere" is divided into 88 constellations, which were chosen essentially arbitrarily, based largely on historically significant apparent groupings of stars.
Stars in a constellation superficially look to us as though they're near each other, but they may be very different distances from us, because we see the night sky as two-dimensional.
For example, one of the furthest stars visible to the naked eye is over 16,000 light years away, called V762 Cas in the constellation Cassiopeia. Cassiopeia also contains many other stars that are much closer to us, such as Eta Cassiopeiae which is just 19.4 light years away, practically a next-door neighbor.
