A globular cluster is a spherical bundle of stars that orbits a galaxy as a satellite. It is very tightly gravitationally bound, which gives it the spherical shape, and extremely dense (in relative terms) towards its core. Globular clusters are usually composed of hundreds of thousands of old stars, similar to the bulge of a spiral galaxy but confined to a volume of only a few cubic parsecs. Globular clusters are fairly numerous; there are about 150 currently known globulars of the Milky Way (with perhaps 30-50 more undiscovered), and larger galaxies like Andromeda have more (Andromeda may have as many as 500). Some giant elliptical galaxies (e.g., M87) may have as many as 13 to 15 thousand globular clusters!
With a few notable exceptions, each globular cluster appears to have a definite age. That is, all the stars in the cluster formed at virtually the same time. It was the recognition of this fact, studying H-R diagrams of globulars, that led to the earliest understanding of stellar evolution.
Some globular clusters (like Omega Centauri in our Milky Way, and G1 in M31) are truly massive clusters, with several million times the mass of our Sun. Such globular clusters may be the former nuclei of galaxies that once orbited their host galaxy, but were totally engulfed and tidally stripped of their stars save for the dense nucleus. However, most globular clusters are much smaller, having on the order of one hundred thousand stars. Globular clusters are considered part of the halo or spheroid of most galaxies, orbiting the centers of galaxies at distances of up to one hundred kiloparsecs.
It was through the study of globular clusters that the sun's position in the Milky Way galaxy became known. Until the 1930s, it was thought that the sun was near the middle of the galaxy because the distribution of stars in the observable Milky Way appeared uniform. However, the distribution of globular clusters was strongly asymmetric. In looking at the distances to the clusters, it became clear that the observable Milky Way was only a small part of the total galaxy, most of which was obscured by gas and dust. Observing distant objects in the plane of the Milky Way is very difficult because of the dense gas and dust of the galactic disk blocks our view. Consequently, most of the known Milky Way globular clusters lie outside the galactic plane; there are quite possibly a few that lie on the other side of the disk from the Sun, yet undiscovered.
Most globular clusters are very old, and probably formed along with the host galaxy. However, some blue globular clusters have been observed in other galaxies and their blue color is believed to be indicative of hot, young stars and a relatively recent birth. It is not yet known whether globular clusters can naturally form later in the life of a galaxy, but it is likely their formation is tied to catastrophic events such as galaxy mergers.
Globular clusters have a very high star density, and therefore close interactions and near-collisions of stars do sometimes occur. Some exotic classes of stars, such as blue stragglers, millisecond pulsars and low-mass X-ray binaries are much more common in globulars.