A group of stars with higher than average star density for that particular region of space. In the part of the Galaxy where our Sun is situated the star density is relatively low.
Groups between 10 and a couple of hundred stars situated in the same region of the sky; they have irregular shapes, with the stars interacting gravitationally with one another. Over time they become even looser, and can eject stars.
Examples: Pleiades (M45), Hyades (Collinder 50)
A group of tens of thousands up to mullions of stars. They contain some of the youngest stars in our Universe. They are found outside the galactic plane, in the galactic halo.
Examples: M13, M79