An open star cluster is a large group of related stars, which hold themselves together through their overlapping gravitational fields. Unlike the so-called globular star clusters, however, the stars in an open star cluster have begun to drift apart; they are still being held loosely together by gravity, but they have grown substantially in size.
The closest open star cluster to Earth is Hyades, a cluster of several hundred stars located about 150 light-years from our solar system. However, one of the most visible is actually farther away: the Pleiades (Seven Sisters) cluster, about 400 light-years away. Pleiades contains over a thousand member stars, many of them extremely hot (and therefore extremely visible) blue stars. At this distance, only a small number of the cluster can be easily spotted - hence its older name, "Seven Sisters."
One of the key characteristics of an open star cluster is that its stars are of roughly the same age, and roughly the same chemical makeup (the ratio of different elements which make up the stars, usually hydrogen, helium, and a scattering of metals). This is presumed to be because all of the stars formed from the gradual collapse of the same enormous molecular cloud. All stars, and our own solar system, form from such clouds over billions of years; open clusters, however, must have formed from far larger clouds than our own solar system did. Because they all formed from the same molecular cloud, they should also share a similar basic composition.
However, stars in open clusters are also generally of the same age, because of the way the stellar formation process works. The first stars to form from such a large cloud will be very large, making them blue dwarfs. However, blue stars burn through their mass very quickly, and then explode into supernovas. Within less than a hundred million years, this repeated process of short-lived stars growing and exploding will have blown much of the star-forming gas cloud out of the cluster, halting the formation of new stars.
Open clusters are easily spotted because of the light of these blue stars; however, blue stars have very short life-spans, sometimes only a few million years. As a result, for the most part, scientists believe that a cluster with a large number of blue stars is a very young cluster. Older clusters, when they are found, usually contain mostly yellow stars, like our own Sun, which are much longer-lived.
However, one current puzzle is the existence, even in many of these older clusters, of unexpected blue stars. These so-called "blue stragglers" are not predicted by the above theory of open cluster formation: an old cluster stopped forming new stars long ago, and any blue stars which existed should have died out. The current hypothesis is that the blue stragglers formed through collisions of smaller stars - stellar collisions being an extraordinarily rare occurrence in most conditions, but much more likely in the cramped conditions of a star cluster.
Astronomers have located about one thousand clusters in the Milky Way galaxy, including the Pleiades and Hyades clusters mentioned above. They tend to occur where gas clouds are largest and densest, like the spiral arms of the galaxy. In so-called elliptical galaxies, where star formation has more or less come to an end, there are few clusters; those which might once have existed have by now drifted apart as the combined force of the stellar gravity weakened.
In general, however, open star clusters do not survive for very long. Stars are massive objects; as they drift apart, their gravitational effect on each other gradually weakens, until eventually more and more stars escape the system. This further weakens the gravitational hold on the remaining stars, thus accelerating dispersal.
Even those clusters which are large enough to survive for more than a few hundred million years will, sooner or later, pass by another large gravity source (such as a molecular cloud, or another star cluster), be disrupted, and slowly fly apart. In such cases, they may become "moving groups" - stars which are no longer held together in clusters, but are still moving along in roughly the same direction. One moving group is the Ursa Major Moving Group, a collection of stars which once formed the ancient constellation of the Bear, and currently form such constellations as the Big Dipper.