Globular clusters contain from tens of thousands to millions of stars. They are usually spherical or nearly so and range from tens of light-years to about 200 light-years in diameter. All of the stars in a given globular cluster formed at about the same time, and the age of a cluster can be determined by observing the most massive remaining main-sequence stars (if G-class stars are evolving off the main sequence, then the cluster is roughly 10 billion years old).

Globular clusters are fairly common. There are over 150 associated with the Milky Way galaxy, and most other galaxies are also orbited by globular clusters; in fact, the giant elliptical galaxy M87 is surrounded by several thousand globular clusters.
Source(s):
http://messier.obspm.fr/glob.html
http://nedwww.ipac.caltech.edu/level5/ESSAYS/Cudworth/cudworth.html

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