Stone circles are a type of megalithic site. Between 3,000 BCE to 1,200 BCE1 some, unknown, (almost certainly pre-Celtic and therefore
pre-Druidic) people located in the
British Isles and Northern France began erecting rings of stones. These monuments
consisted of at least six stones (or menhirs), half a meter to seven meters tall, in a roughly circular
pattern (actually, true circles were quite rare). It is generally theorized these circles
served as worship sites, astronomical observatories, or perhaps
both. Over 800 sites are known (and in fact one can purchase books listing them, or see maps online).
Stonehenge is the best known example, though there are aspects that make it unique. One of the most
important is the horizontal "lintels" it had. Another was the distance the stones were from the quarry;
in a number of other cases, it would have been trivial to move the stones.
Alexander Thom proposed a number of types, based on geometric construction: True circles, Flattened Circles (of which there
were two types), Egg shapes (two types again), and Ellipses.
Flattened circles were constructed by using smaller circles to to replace a part of the large circle
arc. Egg shapes, on the other hand, required the use of right triangles. These types show sophisticated
thought went into the design of these sites, at least from a placement perspective.
Stone circles tend to attract a lot of New Age associations. They are viewed as sacred sites with great power, or
targets for ley lines. Of course, you also get the Atlantis/Mu/UFOs/IBM were responsible for the
construction of them (especially Stonehenge) bit. Sites with some similarities (particularly in alignment
with celestial occurrences) are found worldwide, but the other sites
tend to be different from a set of standing stones. The various fraternal Druidic groups in England like to
celebrate at them, and while it may be likely the Druids understood the astronomical function of the sites, it is unlikely
they constructed the sites.
Of course, this is based on carbon dating
of nearby plant matter that they think
to the construction of the stones.
Resources: There is a lot of information elsewhere online, as well as books galore. One site with a non-fantastic bent to is http://www.geocities.com/athens/parthenon/6197/.
The Open Directory has a large listing at http://dmoz.org/Science/Social_Sciences/Archaeology/Megaliths/United_Kingdom/.