Quoted from various sources: 

Stonehenge is a megalithic monument on the Salisbury Plain in Southern
England, composed mainly of thirty upright stones (sarsens, each over
ten feet tall and weighing 26 tons), aligned in a circle, with thirty
lintels (6 tons each) perched horizontally atop the sarsens in a
continuous circle. There is also an inner circle composed of similar
stones, also constructed in post-and-lintel fashion.

When sketched in the mid 1700s by William Stukeley, the stones were a
jumble. All the large stone have been uprighted with cranes in the 20th
century, and placed where they might have stood. The lintels have also
been placed across the uprights again.

The archaeology points to a construction date between 5,000 and 3,000
years ago (more than likely, several construction dates over this time).

Stonehenge has a long history of building and remodeling, but
excavation has revealed that there were 3 main periods of building. The
first beginning, about 3100 BC, included the digging of a circular ditch
and ring of 56 pits called the Aubrey Holes.

In the second period, about 1,000 years later, the massive rock pillars
were somehow transported from Southernwestern Wales, and put up in two
distinct concentric rings around the center of the site. It is believed
that this double circle was never finished and was dismantled during the
period of rebuilding.

About 2,000 BC, the first stone circle (which is now the inner circle),
comprised of small bluestones, was set up, but abandoned before
completion. 

Stonehenge was probably finally completed around 1500 BC.