its stone and its a henge.
It is just west of Amesburyon the A 303. If you get to Salisbury there is a regular tour bus, or from London..
See the Related Question.
Stonehenge is a World Heritage Site, with ancient monuments of earthworks and standing stones.
It was added to over some 1500 years (people, not meerkats or aliens built it). See the timeline on the link