Sphinx, here's the website link for RTR:http://www.rometotalrealism.com/preview.htm
and here's the main forum for it where you'll find lots of discussion:http://www.twcenter.net/forums/index.php?showforum=53
The mod balances stats, cost, and upkeep of all units; removes and adds units; reskins most units; modifies some units' sizes and formations; adds many new territories; changes starting assets; changes game events such as marian reform; changes tech trees; adds new music and voiceovers; changes banners; adds ZoR (zone of recruitment) that changes what units you can recruit at freshly conquered provinces based on their original culture.
Basically, it changes everything; it will take you a good hour to just read through all the changes since version one. The upcoming 6.0 release that's planned for mid-may is incredibly ambitious.
In my mind, this is what RTW should have been out of the box; it makes vanilla RTW look shallow and poorly test in comparison.