The term 'henge' is being increasingly used to defined a variety of structures built between 3000-5000 years.  These take the form of banked enclosures, stone chambers and separate stones arranged in avenues and circles.  Some circles have very complex geometry and the local astronomy of the site was often built into the design.
Neolithic farmers appeared first appeared in Britain around 4000BC and by 3000BC the first megalithic (large stone) constructions started. There are several types of stone structures, the simplest being the raising of single, sometimes massive, standing stones called menhirs. Hollow box-like structures made from large stones were used for burials and covered with mounds of soil, these are called dolmens or, in Wales, cromlechs . Cairns are cromlechs covered in stones and barrows are earthen burial mounds. Henges are usually stone or vertical timber circles often surrounded by a ditch and bank.

Most stone circles were constructed in the late Neolithic to early Bronze Age periods. They occur all over the UK except in the southeast where there is no evidence of them, but this does not mean there were none. Many stone circles have been removed and recycled in walls and buildings or cleared for farming. They survive best where there has been little development. Stone circles are also found in parts of Europe.
Early circles, or henges, were made from wood, both Woodhenge and Stonehenge are examples of these. Stone was used for permanence and local materials were generally used but sometimes stones were transported great distances. The proximity of circles to burials suggests they were built with some significance to the dead or to hold ceremonies as well as being "calendars" lined up with astronomical features such as solstice sunrises, moonrise and moonset and specific constellations.
Following detailed surveys of hundreds of sites, Alexander Thom, a professor of engineering at Oxford, concluded that the megalithic builders used a standard unit of measurement.  This unit was 2.72 feet (0.83m), the megalithic yard, about the size of a pace except that over all his surveys he shows there is only a deviation from proportions of this dimension of only 0.003 feet! His findings show the builders used whole or half units of megalithic yards in the radii of stone circles. Many structures include extremely accurate alignments with key solar and lunar risings and settings, often using natural features like mountain peaks, notches in hills or the strategic placement of standing stones as precise sightlines. These astronomical alignments were an integral part of the design of many sites. The geometry of the non circular stone circles was not by accident or inaccuracy but was based on complex ellipse or flattened circle geometric construction.
Solar symbols are common in megalithic tombs, the role of the sun would have been important to Neolithic farmers as crops and cattle depended on it. Stones placed in a circle were sighted to show the occurrence of the summer and winter solstices. In the winter in the northern hemisphere the sun rises in the southeast and stays low in the sky, setting in the southwest. During the spring and summer these positions shift farther north along the horizon and the sun stays in the sky longer. Some tomb passages were aligned so that the rising sun of the winter solstice would shine down them. This would represent the start of the lengthening of the days and the start of the growing season being a cause for celebration.