Open Clusters are groups of stars that exist within the galaxy and are sometimes called galactic clusters. These are groups of stars that form together from a common origin, often from the same cloud of gas.
Open Clusters are classified by their Trumple type which classify their concentration, range of brightness and richness. The concentration refers to how close the member stars appear to each other. The range of brightness refers to the relative brightness between the members. The richness refers the the number of stars in the cluster.
We see open clusters concentrated in the plane of the Milky Way. This makes sense because it is in these areas where the clouds of dust and gas which give rise the clusters are located.