From the World Heritage inscription:
Built in the 10th century, this is the largest temple compound dedicated to Shiva in Indonesia. Rising above the center of the last of these concentric squares are three temples decorated with reliefs illustrating the epic of the Ramayana, dedicated to the three great Hindu divinities (Shiva, Vishnu and Brahma) and three temples dedicated to the animals who serve them.
Prambanan is very similar in architectural style to the early Hindu temples you can see in Angkor, Camboida or My Son, Vietnam. While it is often overshadowed by nearby Borobudur, it is in many respects much more impressive. Unfortunately, an earthquake in 2006 did significant damage to the entire complex. Restoration efforts are underway, but it was pretty obvious that major damage was done to the site.
Prambanan is even closer to the center of Yogjakarta than Borobudur and should be included in any visit to the city. There is also an opera which takes place at in the evening at Prambanan which uses the ruins as a backdrop. I didn’t get to attend, but it looked pretty cool. It is possible (but probably not advisable) to fly to Yogjakarta and visit both Prambanan and Borobudur from Bali and return in one day. Taking a day or two is a good idea and well worth the effort.