Borobudur is not actually a temple per se, which is to say, it is not an edifice to be entered. Instead, it's a huge symmetrical monument, a giant stupa, a commemorative structure which recalls and embodies Buddhist cosmology, as BiggPoppaliscious' cut-and-paste says. It is not far from the lovely town of Yogyakarta on the island of Java, and I recommend a visit to that town, as well as to Borobudur itself.
Borobodur was built on a hill which was terraced and faced with rock; the stone was then elaborately carved. The huge stupa was begun at a period when Hinduism held sway in Java, and the lower 2 levels, which I believe are square, not rectangular, most clearly reflect Hindu mythology. Construction on these levels was begun around 775 AD. A period of turmoil on the island followed; Buddhist leaders gained power, and the monument's significance was diverted to Buddhist ends. Construction ended around 850 AD. Later, Islam swept the island, and most people on Java today are Muslim; Borobudur consequently fell into disrepair, but it remains a visible reminder of this earlier, Buddhist past.
It is odd to say that the stupa on top is "symbolically empty". In fact it's really empty, to symbolize Nirvana, the cessation of all thought, emotion, and action.
A Hindu-Buddhist syncretic religion is still practiced on neighbouring Bali, one reason why that island is so unique and peaceful.
BiggPoppaliscious is incorrect in thinking that s/he stood "on stones that were placed there thousands of years ago", for the stupa is not that old. In addition, when Islam took hold on Java, Borobudur fell into ruin, and jungle grew over the rubble. European hubris relates that the monument was "discovered" by Sir Thomas Stamford Raffles in 1814. When you see such locutions, think "discovered by Europeans". Such a huge structure, even if crumbled and overgrown, could hardly have escaped the notice of the people who lived around it at the time. Europeans undertook reconstruction of the amazing stupa, a mind-numbingly complex task which relied, no doubt, on plentiful native labour and expertise. Today Borobodur is a truly impressive rebuilt structure that looms over the Muslim villages around it as even its rubble must have done a few centuries ago.