A globular cluster describes, as the name already suggests, a spherical cluster of several 100.000 stars that gravitate towards each other. A globular cluster itself is bound to other galaxies by mass attraction.
Due to the high density of stars, even a very good telescope cannot detect individual stars in the center of the globular clusters. In their periphery (their closet neighborhood), however, individual stars can be recognized.
In contrast to open star clusters, globular clusters consist mainly of red, old stars. It is suspected that the oldest stars of the universe are located in those clusters. Due to the absence of interstellar gases, new stars can not be formed in a globular cluster. This, in turn, means that it is a "closed society" whose components getting older exclusively and do not produce new stars. The greater the mass of a star the shorter its lifetime. Therefore, only relatively small stars can be found in the old globular clusters.
Globular clusters appear in heaps in the universe. Within the halo of the Milky Way 150 clusters have already been discovered. In 1665, a German amateur astronomer identified the first globular cluster. At that time Johann Abraham Ihle recognized only a round spot and no single stars due to the bad resolution of his telescope.
Regarding the formation of globular clusters, it is assumed that large gas clouds contracted into galaxies due to gravity.