Generally speaking it's any object in the sky that isn't a star or in our solar system. Most deep sky objects are faint, diffuse and require a telescope to see. They make for spectacular photographs but often appear as little more than a faint smudge of light to your eye, even as seen through a large telescope. These are loose groupings of many stars.
Some open clusters are large and spread out.
For instance, the Hyades, Pleiades and the Beehive are all open clusters that you can see with your naked eye. Others are small and faint. If the stars are very faint the cluster may look more like a hazy patch of sky in your telescope rather than individual stars. When you can make out the individual stars we say the cluster is said to be resolved . Some open clusters have only a few member stars, while the hundreds of stars in others look like jewels spread out across the field of view.