These are groups of dozens to hundreds of stars, which formed together and are continuing to travel the galaxy together. They are usually relatively young (from a few million to a few hundred million years) and still have lots of bright stars. Most of the clusters we see are in our galactic neighborhood, in the spiral arms.

Observing Open Clusters:

• naked-eye: a few are visible as fuzzy patches in the sky - Pleiades, Hyades, Beehive
• binoculars: show spectacular bursts of stars where the naked eye sees little or nothing
• telescopes: the effect is often lost because the whole cluster doesn't fit in the field of view;
  generally, use a low-power, wide-field eyepiece

Best Bets:
There are many open clusters available to show, including 27 of the Messier objects. However, only a few of them are beautiful and impressive to most of our visitors. The fainter open clusters are an acquired taste; for the beginning stargazer, there's not much difference between an open cluster and any other bunch of stars in a field of view. Here are some especially pleasing open clusters.
(Links are to the SEDS website description.)

More open clusters to try out. M6 & M7 are recommended

Note: the "number of stars" data is wildly variable - depends if you're counting by those visible in amateur scopes, or all the stars that have been determined to belong to the cluster in professional-level studies. Many of the numbers given here are for stars brighter than 13 mag, many more than we will actually see with amateur scopes and binoculars.

Data sources:

SEDS "Open Clusters" website: www.seds.org/messier/open.html
(note on ages: if several given, I've used the one attributed to Sky Catalog 2000)

Sky and Telescope Messier Card, 1997

Watson, Brent. Finder Charts of the Messier Objects, 1993

(Monthly availability determined from star charts in Rey, The Stars: a New Way to See Them and at Heavens-Above.com)