• If you're just interested in that Indiana Jones building, The Treasury, Petra, Jordan has a photo and the very briefest of introductions
• If it's the kids who want to learn about the city, try Petra: The Red Rock City from The Knowledge Adventure children's encyclopedia
• Petra: The Drama of History at MIT has some good overall information, both from a tourist standpoint and from a historical standpoint; several very nice pictures and a movie clip (I'm not sure if it's from the Indiana Jones movie or not)
• A Tour Guide to Petra: good touristy description, with two photos
• Focus on Petra: another good touristy description, with photos interspersed in the text
• Petra History and Location makes better use of hypertext than the other sites here (it has links to other sections on different pages instead of one long document) and includes a clickable map which will give you some idea where things are
• A Brief History And Some Photographs claims to be information from a pamphlet bought in a gift shop there at the city; this site has LOTS of good pictures

I do realize that the city of Petra has very little to do with rock band Petra, but what the heck... it can't hurt to learn something new! And, as a matter of fact, there are people who believe that the city Petra is where the Lord will return to. That is not my personal belief, but I've been wrong before...