They appear as one, because the light merges together. The way astronomers find these stars is by detecting wobbles in the stars and differences in orbits and wavelengths. Star clusters, also called star clouds, a clusters of stars that are very close together. To the naked eye, they may appear is a irregular, bright clouds. But through telescopes, they are easily visible, that is the stars in the cluster.
Open clusters are exactly that, open.
The stars are spread out, so you can see some space between them. Closed clusters are closed. This is why, cluster appears as a ball. With a telescope, the outer stars are well visible. A star is a luminous and gaseous body that makes energy by nuclear fusion reactions, which occur in the core.