Among the attractive celestial targets in the night sky are the star clusters. Star clusters are said to be formed in interstellar clouds when clumps of these material begin to undergo gravitational collapse. The collapsing material in turn form stars and other celestial bodies until it is consumed or blown away by stellar winds (a small amount can be retained and can be seen as nebulosity). The stars that form from a single cloud emerge as a cluster of stars. These stars either stay together in one group, or slowly drift apart from one another. This is why some clusters are very loose, while others are tightly distributed. Open clusters are important in studying stellar evolution. The stars that emerge in the cluster are typically of the same age, distance and chemical properties. In the Milky Way alone, there are roughly 1600 confirmed open clusters out of the 50-100,000 suspected. Some of the famous open clusters are the Beehive Cluster [M44], the Jewel Box,and the Pleiades cluster [M45].
A nice analogy for open clusters is the use of bubbles. Bubbles allow us to imagine the formation of stars in an open cluster. As the bubbles for in a chain of loops held together by surface tension, they resemble areas of clumped up matter - kinda like the famous "pillars of creation" image in the Eagle Nebula.
The chain eventually breaks apart and forms into individual bubble structures which we can use as an analogy for the stars. The stars/bubbles are originally confined together into a small area (tight clusters) and later spread out in space (loose cluster). Some bubbles that appear to stick together can be compared to stars that form binary pairs as a result of gravitational interaction.