I'm providing you the Wikepedia page up front for the Pantheon. http://en.wikipedia.org/wiki/Pantheon,_Rome
Frankly, I feel stupid that I go visit these sites, but still don't know much about them. They are gorgeous and imposing, but the signs explaining their significance are few - and they don't have audiotours to rent like in Dutch museums. I would have to hire a live guide or read myself. I figure that Wikipedia is good enough for my purposes.
The other thing is that I am so careful of pickpockets when reading the placcards or visiting, that it is distracting. Pickpockets in Rome make pickpockets the world 'round look like amateurs. I don't carry much cash, but don't want my snazzy new camera to go missing.
Mostly, what I am noticing here in Rome is that everything is so old and ornate and amazing, that it is almost impossible to really capture the history in a short tidbit. The Patheon is a good example. It started out as a pagan temple pre-Roman empire and is now a catholic church. There is layer upon layer upon layer of history in almost anything in Rome.
These last two photographs are interesting. The one on the left is of a tomb of royalty, buried in the 20th Century, including Queen Margherita, who Pizza Margherita is named after. The picture on the right is a small portion of the church that seemed to be left with the original temple walls, etc., and not fitted out as a Catholic chapel with marble, paintings, statues, et al.