If you have ever seen a discussion of Druids in the popular media, you probably saw pictures or video of Druids gathering at Stonehenge. The hidden implication of these depictions is that the Druids built Stonehenge. The Druids were a Celtic priesthood and class, and the problem with such implications is that most historians and anthropologists agree that the Celts did not arrive in Britain until around 500 BCE. Stonehenge was probably built around 1550 BCE, over a thousand years before the Celts, and therefore the first Druids, would have arrived.
On the other hand, The Celts were more of a culture and a language group than a race of people. It could be that Stonehenge was built by an earlier people, and when the Celts arrived on the British Isles, they incorporated the beliefs of those earlier people into their own spiritual practices. The Druids were and are attuned to the cycles of nature, and Stonehenge is nothing if not a place to mark and celebrate the passing seasons. So they may have understood and appreciated its significance and taken it for their own, just as Christians built churches and cathedrals on older Pagan sites.
The association of the Druids with Stonehenge probably began during the Druid Revival of the 18th century, when historians of the period had a tendency to associate anything mysterious with the Druids. This association may or may not be erroneous. Time will tell as more archaeological evidence becomes available. What is certain is that the modern pairing of the Druids and Stonehenge has now been indelibly stamped into the collective consciousness. This can be demonstrated by the fact that modern Druids celebrate the solstices at Stonehenge. Some groups have even replicated this enigmatic stone circle in places as diverse as Washington State, Missouri, and New Zealand.