There are many parts to the Imam Reza Shrine Complex. One part is a mausoleum of the eighth Shia Imam, another a library, and another the Ghoharshad mosque, which is the largest mosque in the world by dimension and the second largest in the world by capacity.
This whole complex is dazzling and beauteous, with nuemrous domes and minarets, showing off colors of blue and gold. Be respectful while touring these grounds, as you are in one of the most holy places in Shia Islam. You should dress conservative, and women should wear chador. Additionally, no bags or luggage is allowed inside, nor are cameras (though you can use the camera on your mobile phone).