Depending on the site, their construction has been dated back to anywhere between five and two thousand years b.c. Many of the ones in Europe are closer to the earlier date and are the oldest examples of European architecture. Their builders are covered by the term 'protohistoric,' meaning 'just before recorded history' . We know very little about them except that they spent an inordinate amount of time pushing very heavy stones into place. Both words are from Breton, a Celtic language spoken in an area of northern France where a great many of them have been found.
Dolmen means 'table stone' and menhir 'tall stone'.
Thus, dolmen refers to a chambered construction, generally a flat 'table stone' supported on three sides by upright slabs, with the fourth side open as an entrance to the chamber. In some cases there may also be a covered corridor leading into the chamber through the open side. The smaller stone slabs arrayed on either side of the entrance define the corridor leading to the main chamber. Originally, dolmens served as tombs, either for individuals or groups, and both chamber and entrance were covered over with cairns or earth. Although dolmens clearly had a funerary purpose and, hence, probably played a significant part in religious rituals and other ceremonies of the Neolithic peoples who built them, there is much less unanimity of opinion on the function of free-standing menhirs, such as the circle at Stonehenge or those on the Orcadi islands of Scotland.